This is a complete refactoring of the bluetooth code.

Now that BlueZ 4 support was dropped, the separation of NMBluezManager
and NMBluez5Manager makes no sense. They should be merged.

At that point, notice that BlueZ 5's D-Bus API is fully centered around
D-Bus's ObjectManager interface. Using that interface, we basically only
call GetManagedObjects() once and register to InterfacesAdded,
InterfacesRemoved and PropertiesChanged signals. There is no need to
fetch individual properties ever.

Note how NMBluezDevice used to query the D-Bus properties itself by
creating a GDBusProxy. This is redundant, because when using the ObjectManager
interfaces, we have all information already.

Instead, let NMBluezManager basically become the client-side cache of
all of BlueZ's ObjectManager interface. NMBluezDevice was mostly concerned
about caching the D-Bus interface's state, tracking suitable profiles
(pan_connection), and moderate between bluez and NMDeviceBt.
These tasks don't get simpler by moving them to a seprate file. Let them
also be handled by NMBluezManager.

I mean, just look how it was previously: NMBluez5Manager registers to
ObjectManager interface and sees a device appearing. It creates a
NMBluezDevice object and registers to its "initialized" and
"notify:usable" signal. In the meantime, NMBluezDevice fetches the
relevant information from D-Bus (although it was already present in the
data provided by the ObjectManager) and eventually emits these usable
and initialized signals.
Then, NMBlue5Manager emits a "bdaddr-added" signal, for which NMBluezManager
creates the NMDeviceBt instance. NMBluezManager, NMBluez5Manager and
NMBluezDevice are strongly cooperating to the point that it is simpler
to merge them.

This is not mere refactoring. This patch aims to make everything
asynchronously and always cancellable. Also, it aims to fix races
and inconsistencies of the state.

- Registering to a NAP server now waits for the response and delays
  activation of the NMDeviceBridge accordingly.

- For NAP connections we now watch the bnep0 interface in platform, and tear
  down the device when it goes away. Bluez doesn't send us a notification
  on D-Bus in that case.

- Rework establishing a DUN connection. It no longer uses blocking
  connect() and does not block until rfcomm device appears. It's
  all async now. It also watches the rfcomm file descriptor for
  POLLERR/POLLHUP to notice disconnect.

- drop nm_device_factory_emit_component_added() and instead let
  NMDeviceBt directly register to the WWan factory's "added" signal.

The previous function arguments of nm_modem_act_stage2_config() act as if the
function could fail or even postpone the action. It never did.

We cannot treat this generic. A caller needs to know whether nm_modem_act_stage2_config()
can postpone the action, and when it does, which signal is emitted upon completion. That
is, the caller needs to know how to proceed after postponing.

In other words, since this function currently cannot fail or postpone
the stage, so must all callers already rely on that. At this point it makes
no sense to pretend that the function could be any different, if all callers
assume it is not. Simplify the API.

Currently, we cannot ask which modems exist. NMDeviceBt may claim it
via nm_device_factory_emit_component_added(), and NMWWanFactory may
take it by listening to NM_MODEM_MANAGER_MODEM_ADDED. But that's it.

We will drop nm_device_factory_emit_component_added() because it's only
used for passing modems to NMDeviceBt. Instead, NMDeviceBt can directly
subscribe to NM_MODEM_MANAGER_MODEM_ADDED. It already has a reference
to NMModemManager.

Anyway, the NM_MODEM_MANAGER_MODEM_ADDED signal is no enough, because
sometimes when the mode appears, NMDeviceBt might not yet know whether
it should take it (because the DUN connect call is not yet complete).
Currently that never happens because dun_connect() blocks waiting for
the device. That must be fixed, by not waiting. But this opens up a
race, and NMDeviceBt might after NM_MODEM_MANAGER_MODEM_ADDED need to
search for the suitable modem: by iterating the list of all modems.

NMModem-s are either used by NMDeviceModem or by NMDeviceBt.
The mechanism how that is coordinated it odd:

  - the factory emits component-added, and then NMDeviceBt
    might take the device (and claim it). In that case, component-added
    would return TRUE to indicate that the modem should not be also
    used by NMDeviceModem.

  - next, if the modem has a driver that looks like bluetooth, NMDeviceModem
    ignores it too.

  - finally, NMDeviceModem claims the modem (which is now considered to
    be non-bluetooth).

I think the first problem is that the device factory tries to have this
generic mechanism of "component-added". It's literally only used to
cover this special case. Note that NMDeviceBt is aware of modems. So,
abstracting this just adds lots of code that could be solved better
by handling the case (of giving the modem to either NMDeviceBt or
NMDeviceModem) specifically.

NMWWanFactory itself registers to the NM_MODEM_MANAGER_MODEM_ADDED
signal and emits nm_device_factory_emit_component_added().
We could just have NMWWanFactory and NMDeviceBt both register to
that signal. Signals even support priorities, so we could have
NMDeviceBt be called first to claim the device.

Anyway, as the modem can only have one owner, the modem should have
a flag that indicates whether it's claimed or not. That will allow
multiple components all look at the same modem and moderate who is
going to take ownership.

This allows the GSM connection to Just Work most of the time, as in:
"nmcli d connect ttyUSB0".

Though there is no known way to reach these, it may well happen that
this is going to prevent some hard crashes.

Just a handful of unit tests.

This allows up to look up a default APN if the user doesn't pick one.

Fixes: 7a72c705acc2 ('initrd: add devicetree support')

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