relative paths in the DB or URLs.  The main functional difference is that
playlistmove and playlistdelete will rewrite playlists in the correct
encoding and remove invalid lines instead of potentially modifying them.
The specific changes are:

appendSongToStoredPlaylist:
* Don't convert to FS charset
* Don't prepend music_directory if saving absolute paths

writeStoredPlaylistToPath:
* Convert to FS charset
* Prepend music_directory if saving absolute paths

loadStoredPlaylist:
* Make sure each line is either in the DB or a URL

loadPlaylist:
* Don't bother checking paths, since it's done in loadStoredPlaylist now

git-svn-id: https://svn.musicpd.org/mpd/trunk@6266 09075e82-0dd4-0310-85a5-a0d7c8717e4f