queue: duplicate all song objects

Make sure the queue "owns" all song objects, so nobody else can free
them.

test/FakeSong.cxx

@@ -0,0 +1,33 @@
+/*
+ * Copyright (C) 2003-2012 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "config.h"
+#include "song.h"
+#include "directory.h"
+#include "gcc.h"
+
+#include <stdlib.h>
+
+struct directory detached_root;
+
+struct song *
+song_dup_detached(gcc_unused const struct song *src)
+{
+	abort();
+}