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.c

@@ -40,6 +40,8 @@ queue_next_order(const struct queue *queue, unsigned order)
 {
 	assert(order < queue->length);
 
+	if (queue->smartstop)
+		return -1;
 	if (order + 1 < queue->length)
 		return order + 1;
 	else if (queue->repeat)
@@ -275,6 +277,7 @@ queue_init(struct queue *queue, unsigned max_length)
 	queue->version = 1;
 	queue->repeat = false;
 	queue->random = false;
+	queue->smartstop = false;
 
 	queue->items = g_new(struct queue_item, max_length);
 	queue->order = g_malloc(sizeof(queue->order[0]) *