settings: don't implement settings plugins as singletons

The settings plugins are created by NMSettings when the plugin
gets loaded. There is no need for these instances to be singletons
or to have a singleton getter.

Also, while in practice we create a settings plugin instance of
each type only once, there is nothing that would prevent creating
multiple instances. Hence, having a singleton getter is not right.

What is however useful, is to track them and block shutdown
via nm_shutdown_wait_obj_register*(). While the actual waiting
is not yet implemented, we should mark the plugin instances to
block shutdown (in the future).

src/settings/nm-settings.c

@@ -1255,6 +1255,10 @@ add_plugin (NMSettings *self,
 
 	priv->plugins = g_slist_append (priv->plugins, g_object_ref (plugin));
 
+	nm_shutdown_wait_obj_register_full (G_OBJECT (plugin),
+	                                    g_strdup_printf ("%s-settings-plugin", pname),
+	                                    TRUE);
+
 	_LOGI ("Loaded settings plugin: %s (%s%s%s)",
 	       pname,
 	       NM_PRINT_FMT_QUOTED (path, "\"", path, "\"", "internal"));