playlist: fix stored playlist modifications with absolute paths

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.

src/mapper.h

@@ -32,6 +32,14 @@ void mapper_init(void);
 
 void mapper_finish(void);
 
+/**
+ * Determines the absolute file system path of a relative URI.  This
+ * is basically done by converting the URI to the file system charset
+ * and prepending the music directory.
+ */
+const char *
+map_uri_fs(const char *uri, char *buffer);
+
 /**
  * Determines the file system path of a directory object.
  *