directory: added "mtime" property

Remember the modification time of each directory.  This is important
for archives (which are virtual directories right now), but may also
be useful for an automatic update mechanism.

src/directory.h

@@ -33,6 +33,7 @@ struct directory {
 	struct dirvec children;
 	struct songvec songs;
 	struct directory *parent;
+	time_t mtime;
 	ino_t inode;
 	dev_t device;
 	unsigned stat; /* not needed if ino_t == dev_t == 0 is impossible */