playlist: renamed global "playlist" variable to "g_playlist"

Export the "g_playlist" variable, and pass it to all playlist
functions.  This way, we can split playlist.c easier into separate
parts.  The code which initializes the singleton variable is moved to
playlist_global.c.

src/dbUtils.c

@@ -168,7 +168,7 @@ int printAllIn(struct client *client, const char *name)
 static int
 directoryAddSongToPlaylist(struct song *song, G_GNUC_UNUSED void *data)
 {
-	return addSongToPlaylist(song, NULL);
+	return addSongToPlaylist(&g_playlist, song, NULL);
 }
 
 struct add_data {