add state_file to config, save running playlist over reboots

package/mpd/files/mpd.conf

@@ -2,6 +2,7 @@
 user				"mpd"
 music_directory			"/music"
 playlist_directory		"/etc/mpd/playlists"
+state_file			"/etc/mpd/state"
 db_file				"/etc/mpd/database"
 pid_file			"/var/run/mpd/mpd.pid"
 log_file			"syslog"