playlist: pass struct client to loadPlaylist()

The function loadPlaylist() wants to report incremental errors to the
client, for this reason we cannot remove its protocol dependency right
now.  Instead, make it use the client struct instead of the raw file
descriptor.

src/command.c

@@ -575,7 +575,7 @@ static int handleLoad(struct client *client, mpd_unused int *permission,
 {
 	enum playlist_result result;
 
-	result = loadPlaylist(client_get_fd(client), argv[1]);
+	result = loadPlaylist(client, argv[1]);
 	return print_playlist_result(client, result);
 }