SongUpdate: use OpenArchiveInputStream() to open archive file

Don't pass Path::c_str() to tag_stream_scan() which requires a UTF-8
string.

src/TagArchive.hxx

@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2003-2015 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.
+ */
+
+#ifndef MPD_TAG_ARCHIVE_HXX
+#define MPD_TAG_ARCHIVE_HXX
+
+#include "check.h"
+
+class Path;
+struct tag_handler;
+
+/**
+ * Scan the tags of a song file inside an archive.  Invokes matching
+ * decoder plugins, but does not invoke the special "APE" and "ID3"
+ * scanners.
+ *
+ * @return true if the file was recognized (even if no metadata was
+ * found)
+ */
+bool
+tag_archive_scan(Path path, const tag_handler &handler, void *handler_ctx);
+
+#endif