OTHER: Don't treat "no position" in playlist as an error.

current_id from the output object will return 0 if no song has been
initialized yet, so this change makes them behave the same way.

Additional motivation for this is that the broadcast version of the
current_pos command emits -1 when there's no active position in the
playlist.
OTHER: Always handle playlist position as signed int.
OTHER: Use XMMS_CALL_CHAIN for playlist_length_get/currpos_get.
OTHER: Sort output of 'xmms2 server config'.
OTHER: For 'xmms2 add', if no position, assume append.
OTHER: Add test checks for playlist position on _replace.