Introduce functionality for syncing with the media services

* The global synchro semaphore is provided with the purpose of
being used to avoid threads lock up when the media_server is in
an undefined state. There's still room for improvements.
* BMediaRoster::SyncToServices lock up on a semaphore until
the multi_audio correctly connected to the mixer.

headers/os/media/MediaRoster.h

@@ -42,7 +42,11 @@ public:
 				// same time.
 
 	// Check if the media services are running.
-	static bool					IsRunning();
+	static	bool				IsRunning();
+
+	// This functions blocks until the media services are available,
+	// don't abuse of it.
+	static	status_t			SyncToServices(bigtime_t timeout = -1);
 
 	// Getting common instances of system nodes:
 			status_t			GetVideoInput(media_node* _node);

headers/private/media/ServerInterface.h

@@ -28,6 +28,9 @@ enum {
 	// add_system_beep_event()
 	MEDIA_SERVER_ADD_SYSTEM_BEEP_EVENT,
 
+	// sent by the rescan thread
+	MEDIA_SERVER_RESCAN_COMPLETED,
+
 	// media add-on server
 	MEDIA_ADD_ON_SERVER_PLAY_MEDIA = '_TRU'
 };
@@ -370,6 +373,7 @@ struct server_register_app_request : request_data {
 };
 
 struct server_register_app_reply : reply_data {
+	sem_id					global_synchro;
 };
 
 struct server_unregister_app_request : request_data {