For internal checks (i.e. not in command.c), we need to check whether
an URI is in the databse, in the local file system or a remote URI
with a scheme.
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.
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.
