SongUpdate: use the Storage interface, support remote files

This commit finally allows the database update to scan remote files,
e.g. using the smbclient storage plugin.  However, it is not yet
possible to configure that, therefore the feature is not accessible
yet.

src/db/Song.hxx

@@ -32,6 +32,7 @@
 struct LightSong;
 struct Directory;
 class DetachedSong;
+class Storage;
 
 /**
  * A song file inside the configured music directory.
@@ -91,11 +92,12 @@ struct Song {
 	 * data, nullptr is returned.
 	 */
 	gcc_malloc
-	static Song *LoadFile(const char *path_utf8, Directory &parent);
+	static Song *LoadFile(Storage &storage, const char *name_utf8,
+			      Directory &parent);
 
 	void Free();
 
-	bool UpdateFile();
+	bool UpdateFile(Storage &storage);
 	bool UpdateFileInArchive();
 
 	/**

src/db/update/Archive.cxx

@@ -65,7 +65,7 @@ UpdateWalk::UpdateArchiveTree(Directory &directory, const char *name)
 		Song *song = directory.FindSong(name);
 		db_unlock();
 		if (song == nullptr) {
-			song = Song::LoadFile(name, directory);
+			song = Song::LoadFile(storage, name, directory);
 			if (song != nullptr) {
 				db_lock();
 				directory.AddSong(song);

src/db/update/UpdateSong.cxx

@@ -61,7 +61,7 @@ UpdateWalk::UpdateSongFile2(Directory &directory,
 	if (song == nullptr) {
 		FormatDebug(update_domain, "reading %s/%s",
 			    directory.GetPath(), name);
-		song = Song::LoadFile(name, directory);
+		song = Song::LoadFile(storage, name, directory);
 		if (song == nullptr) {
 			FormatDebug(update_domain,
 				    "ignoring unrecognized file %s/%s",
@@ -79,7 +79,7 @@ UpdateWalk::UpdateSongFile2(Directory &directory,
 	} else if (info.mtime != song->mtime || walk_discard) {
 		FormatDefault(update_domain, "updating %s/%s",
 			      directory.GetPath(), name);
-		if (!song->UpdateFile()) {
+		if (!song->UpdateFile(storage)) {
 			FormatDebug(update_domain,
 				    "deleting unrecognized file %s/%s",
 				    directory.GetPath(), name);