Now that we do proper locking and signalling instead of continuous
polling for IPC, a deadlock was found: at the end of a song, the
player thread waits until the main thread sets pc->queueState from
PLAYER_QUEUE_DECODE to PLAYER_QUEUE_PLAY.  He is never woken up, since
syncPlaylistWithQueue() does not activate the notification.  I added
wakeup_player_nb() to setQueueState(), since the player must be
signalled whenever pc->queueState changes.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7303 09075e82-0dd4-0310-85a5-a0d7c8717e4f