Server never sets the path, so this is entirely unused server-side.
Also NMConnection is a glib interface and stores it's private date
in the GObject's data. It's less efficient to look it up. Just
avoid it.

Contary to nm_simple_connection_new_from_dbus(), this internal
function allows to specify parse-flags.

- All internal source files (except "examples", which are not internal)
  should include "config.h" first. As also all internal source
  files should include "nm-default.h", let "config.h" be included
  by "nm-default.h" and include "nm-default.h" as first in every
  source file.
  We already wanted to include "nm-default.h" before other headers
  because it might contains some fixes (like "nm-glib.h" compatibility)
  that is required first.

- After including "nm-default.h", we optinally allow for including the
  corresponding header file for the source file at hand. The idea
  is to ensure that each header file is self contained.

- Don't include "config.h" or "nm-default.h" in any header file
  (except "nm-sd-adapt.h"). Public headers anyway must not include
  these headers, and internal headers are never included after
  "nm-default.h", as of the first previous point.

- Include all internal headers with quotes instead of angle brackets.
  In practice it doesn't matter, because in our public headers we must
  include other headers with angle brackets. As we use our public
  headers also to compile our interal source files, effectively the
  result must be the same. Still do it for consistency.

- Except for <config.h> itself. Include it with angle brackets as suggested by
  https://www.gnu.org/software/autoconf/manual/autoconf.html#Configuration-Headers

Before we would just call verify() and only return valid connections
without attempting to fix them.

It is better to use normalize(), because that function is especially there to
accept and repair deprecated configurations that would no longer verify().

This changes behavior in the way that the function now accepts connections
that would have been rejected before.

Since commit b88715e05bc16c40ae68958c58aa86c2ad029a82 normalize() also
adds a missing UUID. Hence this also affects the DBUS method 'AddConnection'
in that it now accepts connections without UUID. Previously, clients were
required to set a UUID for the new connection, now NM core can create a random
one if no UUID is set.

https://bugzilla.gnome.org/show_bug.cgi?id=740813

config.h should be included from every .c file, and it should be
included before any other include. Fix that.

(As a side effect of how I did this, this also changes us to
consistently use "config.h" rather than <config.h>. To the extent that
it matters [which is not much], quotes are more correct anyway, since
we're talking about a file in our own build tree, not a system
include.)

In preparation for porting to GDBus, make nm_connection_to_dbus(),
etc, represent connections as GVariants of type 'a{sa{sv}}' rather
than as GHashTables-of-GHashTables-of-GValues.

This means we're constantly converting back and forth internally, but
this is just a stepping stone on the way to the full GDBus port, and
all of that code will go away again later.

libnm-util's connection deserializing/copying/replacing functions have
odd semantics where sometimes they both modify a connection AND return
an error. libnm-core tried to improve things by guaranteeing that the
connection would not be modified if the new settings were invalid, but
this ended up breaking a bunch of places that needed to be able to
work with invalid connections. So re-fix the functions by reverting
back to the old semantics, but having return values that clearly
distinguish whether the connection was modified or not.

For comparison:

  - nm_connection_new_from_hash() / nm_simple_connection_new_from_dbus():

      - libnm-util: returns a valid connection or NULL.

      - OLD libnm-core: returned a valid connection or NULL.

      - NEW libnm-core: returns a valid connection or NULL.

  - nm_connection_duplicate() / nm_simple_connection_new_clone():

      - libnm-util: always succeeds, whether or not the connection is
        valid.

      - OLD libnm-core: returned a valid connection or NULL

      - NEW libnm-core: always succeeds, whether or not the connection
        is valid.

    - nm_connection_replace_settings_from_connection():

      - libnm-util: always replaces the settings, but returns FALSE if
        the connection is now invalid.

      - OLD libnm-core: either replaced the settings and returned TRUE
        (if the settings were valid), or else left the connection
        unchanged and returned FALSE (if not).

      - NEW libnm-core: always replaces the settings, and has no
        return value. (The modified connection is valid if and only if
        the replaced-from connection was valid; just like with the
        libnm-util version.)

    - nm_connection_replace_settings():

      - libnm-util: returns TRUE if the new settings are valid, or
        FALSE if either (a) the new settings could not be deserialized
        and the connection is unchanged, or (b) the new settings were
        deserialized, and the connection was updated, but is now not
        valid.

      - OLD libnm-core: either replaced the settings and returned TRUE
        (if the settings were valid), or else left the connection
        unchanged and returned FALSE (if not).

      - NEW libnm-core: returns TRUE if the connection was updated
        (whether or not it is valid), or FALSE if the new settings
        could not be deserialized and the connection is unchanged.

Add _nm_setting_class_add_dbus_only_property(), for declaring
properties that appear in the D-Bus serialization, but which don't
correspond to GObject properties.

Since some property overrides will require examining settings other
than the setting that they are on (eg, the value of
802-11-wireless.security depends on whether an
NMSettingWirelessSecurity setting is present, and
NMSettingConnection:interface-name might sometimes be set from, eg,
bond.interface-name), we also update _nm_setting_to_dbus() to take the
full NMConnection as an argument, and _nm_setting_new_from_dbus() to
take the full connection hash.

Additionally, with some deprecated properties, we'll want to validate
them on construction, but we don't need to keep the value around after
that. So allow _nm_setting_new_from_dbus() to return a verification
error directly, so we don't need to store the value until the verify()
call.

On failure, nm_connection_replace_settings() would leave the
connection in an undefined state. Fix it so that either (a) the
settings are replaced and the resulting connection is valid and we
return TRUE, or (b) the connection is untouched and we return FALSE
and an error. (And add a test case for this.)

Rename nm_connection_to_hash() to nm_connection_to_dbus(), and
nm_connection_new_from_hash() to nm_connection_new_from_dbus(). In
addition to clarifying that this is specifically the D-Bus
serialization format, these names will also work better in the
GDBus-based future where the serialization format is GVariant, not
GHashTable.

Also, move NMSettingHashFlags to nm-connection.h, and rename it
NMConnectionSerializationFlags.