Modify player-to-DACP status update communication

Have DACP set a callback (through a sync_command() setter) into the player
instead of setting an fd without any locking. All the code now lies in DACP
instead of being split between DACP and the player.

src/player.h

@@ -46,6 +46,7 @@ struct player_status {
 };
 
 typedef void (*spk_enum_cb)(uint64_t id, const char *name, int selected, int has_password, void *arg);
+typedef void (*player_status_handler)(void);
 
 struct player_source;
 
@@ -112,7 +113,7 @@ player_queue_plid(uint32_t plid);
 
 
 void
-player_set_updatefd(int fd);
+player_set_update_handler(player_status_handler handler);
 
 int
 player_init(void);