decoder_plugin: added method container_scan()

[mk: fixed whitespace errors; use delete_song() instead of
songvec_delete()]

src/directory.h

@@ -28,6 +28,7 @@
 #define DIRECTORY_DIR		"directory: "
 
 #define DEVICE_INARCHIVE	(unsigned)(-1)
+#define DEVICE_CONTAINER	(unsigned)(-2)
 
 struct directory {
 	struct dirvec children;