$ find * -type f |xargs perl contrib/scripts/spdx.pl
  $ git rm contrib/scripts/spdx.pl

These functions are now unused. Drop them.

Also, if we ever reintroduce private unix socket, we sure won't use
GDBusProxy. Good riddance.

We no longer add these. If you use Emacs, configure it yourself.

Also, due to our "smart-tab" usage the editor anyway does a subpar
job handling our tabs. However, on the upside every user can choose
whatever tab-width he/she prefers. If "smart-tabs" are used properly
(like we do), every tab-width will work.

No manual changes, just ran commands:

    F=($(git grep -l -e '-\*-'))
    sed '1 { /\/\* *-\*-  *[mM]ode.*\*\/$/d }'     -i "${F[@]}"
    sed '1,4 { /^\(#\|--\|dnl\) *-\*- [mM]ode/d }' -i "${F[@]}"

Check remaining lines with:

    git grep -e '-\*-'

The ultimate purpose of this is to cleanup our files and eventually use
SPDX license identifiers. For that, first get rid of the boilerplate lines.

Note that various components (NMFirewallManager, NMAuthManager,
NMConnectivity, etc.pp) all request their own GDBusConnection from
glib's G_BUS_TYPE_SYSTEM singleton.

In the future, let them instead use the D-Bus connection that
NMDBusManager already has.

- NMDBusManager also uses g_bus_get(G_BUS_TYPE_SYSTEM), so in practice this
  is just the same GDBusConnection instance.

- if it would not be the same GDBusConnection instance, it would
  be more correct/logical to use the one that NMDBusManager uses.

- NMDBusManager already aquired the GDBusConnection synchronously
  and it's ready for use. On the other hand, g_bus_get()/g_bus_get_sync()
  has the notion that getting the singleton cannot be done without
  waiting/blocking. So at least it involves locking or even dispatching
  the async reply on D-Bus.

- in "configure-and-quit=initrd" we really don't have D-Bus available.
  NMDBusManager should control whether the other components use D-Bus
  or not. For example, NMFirewallManager should not ask glib for a
  G_BUS_TYPE_SYSTEM singleton only to later find out that it doesn't work.

So if these components would reuse NMDBusManager's GDBusConnection,
then it must have the connection also in regular "configure-and-quit=true"
mode. In this case, we are in late boot and want do connectivity
checking and talk to firewalld.

We call GetConnectionUnixProcessID and GetConnectionUnixUser *a lot*.
And we do so synchronously. Both is a problem.

To avoid the first problem, cache the last few requests with each cached
value being valid for one second.

On a quick test, this saves 98% of the requests:

     59 GetConnectionUnixProcessID(*)
   3201 GetConnectionUnixProcessID(*) (served from cache)
     59 GetConnectionUnixUser(*)
   3201 GetConnectionUnixUser(*) (served from cache)

Note that now as we serve requests from the cache, it might be the case
that the D-Bus endpoint already disconnected. Previously, the request would
have failed but now we return the cached user-id and process-id. This
problem is mitigated by only caching the values for up to one second.
Also, it's not really a problem because we cache sender names. Those
are supposed to be unique and not repeat. So, even if the peer already
disconnected, it is still true that the corresponding PID/UID was as
we have cached it. We don't use this API for checking whether the peer
is still connected, but what UID/PID it has/had. That answer is still
correct for the cached value after the peer disconnected.

nm_dbus_manager_get_connection() was unused. Also, we already have
nm_dbus_manager_get_dbus_connection() which does the same (and is
used).

The NMDBusManager owns a reference to the system bus. Expose it, so we
can use it. Note that g_bus_get() -- as alternative to get the systembus
singleton -- is asynchronous, and g_bus_get_sync() has an API that makes
one wonder what it does. Since we already have a reference to the connection
object we want to use, expose it.

NMConnection is an interface, which is implemented by the types
NMSimpleConnection (libnm-core), NMSettingsConnection (src) and
NMRemoteConnection (libnm).

NMSettingsConnection does a lot of things already:

  1) it "is-a" NMDBusObject and exports the API of a connection profile
     on D-Bus
  2) it interacts with NMSettings and contains functionality
     for tracking the profiles.
  3) it is the base-class of types like NMSKeyfileConnection and
     NMIfcfgConnection. These handle how the profile is persisted
     on disk.
  4) it implements NMConnection interface, to itself track the
     settings of the profile.

3) and 4) would be better implemented via delegation than inheritance.

Address 4) and don't let NMSettingsConnection implemente the NMConnection
interface. Instead, a settings-connection references now a NMSimpleConnection
instance, to which it delegates for keeping the actual profiles.

Advantages:

  - by delegating, there is a clearer separation of what
    NMSettingsConnection does. For example, in C we often required
    casts from NMSettingsConnection to NMConnection. NMConnection
    is a very trivial object with very little logic. When we have
    a NMConnection instance at hand, it's good to know that it is
    *only* that simple instead of also being an entire
    NMSettingsConnection instance.

    The main purpose of this patch is to simplify the code by separating
    the NMConnection from the NMSettingsConnection. We should generally
    be aware whether we handle a NMSettingsConnection or a trivial
    NMConnection instance. Now, because NMSettingsConnection no longer
    "is-a" NMConnection, this distinction is apparent.

  - NMConnection is implemented as an interface and we create
    NMSimpleConnection instances whenever we need a real instance.
    In GLib, interfaces have a performance overhead, that we needlessly
    pay all the time. With this change, we no longer require
    NMConnection to be an interface. Thus, in the future we could compile
    a version of libnm-core for the daemon, where NMConnection is not an
    interface but a GObject implementation akin to NMSimpleConnection.

  - In the previous implementation, we cannot treat NMConnection immutable
    and copy-on-write.
    For example, when NMDevice needs a snapshot of the activated
    profile as applied-connection, all it can do is clone the entire
    NMSettingsConnection as a NMSimpleConnection.
    Likewise, when we get a NMConnection instance and want to keep
    a reference to it, we cannot do that, because we never know
    who also references and modifies the instance.
    By separating NMSettingsConnection we could in the future have
    NMConnection immutable and copy-on-write, to avoid all unnecessary
    clones.

During shutdown, we will need to still iterate the main loop
to do a coordinated shutdown. Currently we do not, and we just
exit, leaving a lot of objects hanging.

If we are going to fix that, we need during shutdown tell
NMDBusManager to reject all future operations.

Note that property getters and "GetManagerObjects" call is not
blocked. It continues to work.

Certainly for some operations, we want to allow them to be called even
during shutdown. However, these have to opt-in.

This also fixes an uglyness, where nm_dbus_manager_start() would
get the set-property-handler and the @manager as user-data. However,
NMDBusManager will always outlife NMManager, hence, after NMManager
is destroyed, the user-data would be a dangling pointer. Currently
that is not an issue, because
  - we always leak NMManager
  - we don't run the mainloop during shutdown