playlist: added support for adding songs not in the music database

Clients which have authenticated via unix socket may add local files
to the MPD playlist, provided that they own the file.

src/mapper.c

@@ -96,9 +96,12 @@ map_directory_child_fs(const struct directory *directory, const char *name,
 const char *
 map_song_fs(const struct song *song, char *buffer)
 {
-	assert(song->parent != NULL);
+	assert(song_is_file(song));
 
-	return map_directory_child_fs(song->parent, song->url, buffer);
+	if (song_in_database(song))
+		return map_directory_child_fs(song->parent, song->url, buffer);
+	else
+		return utf8_to_fs_charset(buffer, song->url);
 }
 
 const char *