implements the smartstop feature

The smartstop feature is a way to tell mpd to stop playing after
current song.
This patche provides:
- 'state' command returns 'smartstop' state (1 or 0)
- 'smartstop' can activate or not the smartstop state
- when song is terminated, mpd stops playing and smartstop is set to 0

src/queue.h

@@ -81,6 +81,9 @@ struct queue {
 	    reached? */
 	bool repeat;
 
+	/** stop playing after that song. */
+	bool smartstop;
+
 	/** play back songs in random order? */
 	bool random;