If (and only if) there is a current song in the playlist,
(player could be stopped), allow the move destination
argument to be specified as a negative number.

This means moving any song (besides the current one) to the -1
position will allow it to be moved to the next song in the
playlist.  Moving any song to position -2 will move it
to the song after the next, and so forth.

Moving a song to -playlist.length will move it to the song
_before_ the current song on the playlist; so this will
work for repeating playlists, too.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7152 09075e82-0dd4-0310-85a5-a0d7c8717e4f