When save_absolute_paths_in_playlists was enabled in mpd.conf, MPD
broke all playlists when manipulated using the "playlistdelete"
command. The reason was that map_directory_child_fs() was used, which
doesn't accept slashes in the file name. Use the new map_uri_fs()
function instead.
Make map_directory_child_fs() refuse the names "." and "..". This is
currently the interface where an attacker may inject a manipulated
path (through the "update" command).
The mapper library maps directory and song objects to file system
paths. With this central library, the code mixture in path.c should
be cleaned up, and we will be able to add neat features like aliasing.
