notify: protect notify->pending with the mutex

There was a known deadlocking bug in the notify library: when the
other thread set notify->pending after the according check in
notify_wait(), the latter thread was deadlocked.  Resolve this by
synchronizing all accesses to notify->pending with the notify object's
mutex.  Since notify_signal_sync() was never used, we can remove it.
As a consequence, we don't need notify_enter() and notify_leave()
anymore; eliminate them, too.

src/player_thread.c

@@ -384,8 +384,6 @@ static void do_play(void)
 
 static void * player_task(mpd_unused void *arg)
 {
-	notify_enter(&pc.notify);
-
 	while (1) {
 		switch (pc.command) {
 		case PLAYER_COMMAND_PLAY: