This way they will all be able to register with the ObjectManager mock.

When we have the ObjectManager mock, this allows it to emit correct
InterfacesAdded signal when the ExportedObj is initialized.

Add new Reload D-Bus command to reload NetworkManager configuration.

For now, this is like sending SIGHUP to the process. There are several
advantages here:

  - it is guarded via PolicyKit authentication while signals
    can only be sent by root.

  - the user can wait for the reload to be complete instead of sending
    an asynchronous signal. For now, we operation completes after
    nm_config_reload() returns, but later we could delay the response
    further until specific parts are fully reloaded.

  - SIGHUP reloads everything including re-reading configuration from
    disk while SIGUSR1 reloads just certain parts such as writing out DNS
    configuration anew.
    Now, the Reload command has a flags argument which is more granular
    in selecting parts which are to be reloaded. For example, via
    signals the user can:

      1) send SIGUSR1: this writes out the DNS configuration to
         resolv.conf and possibly reloads other parts without
         re-reading configuration and without restarting the DNS plugin.
      2) send SIGHUP: this reloads configuration from disk,
         writes out resolv.conf and restarts the DNS plugin.

    There is no way, to only restart the DNS plugin without also reloading
    everything else.

Fixes: f15c412015647b378a187bdf98ccf8cd75eb0475