core: refactor setting of D-Bus properties via NMManager

- Also if the target object is the NMManager instance itself,
  re-fetch the manager via nm_bus_manager_get_registered_object().
  This way, we only set the property on the manager, if
  it's also exported according to the bus-manager. Also,
  we don't treat the manager instance special.

- Move fetching the object (nm_bus_manager_get_registered_object())
  from do_set_property_check() to prop_set_auth_done_cb(). Otherwise,
  we fetch the object first, but there is no guarantee that the object
  is still exported after the asynchronous authentication succeeds.

src/nm-exported-object.h

@@ -53,6 +53,7 @@ const char *nm_exported_object_get_path    (NMExportedObject *self);
 gboolean    nm_exported_object_is_exported (NMExportedObject *self);
 void        nm_exported_object_unexport    (NMExportedObject *self);
 GSList *    nm_exported_object_get_interfaces (NMExportedObject *self);
+GDBusInterfaceSkeleton *nm_exported_object_get_interface_by_type (NMExportedObject *self, GType interface_type);
 
 G_END_DECLS