directory: update do its work inside a thread

A lot of the preparation was needed (and done in previous
months) in making update thread-safe, but here it is.

This was the first thing I made work inside a thread when I
started mpd-uclinux many years ago, and also the last thing I've
done in mainline mpd to work inside a thread, go figure.

src/directory.h

@@ -34,7 +34,7 @@ typedef struct _Directory {
 	unsigned stat; /* not needed if ino_t == dev_t == 0 is impossible */
 } Directory;
 
-void readDirectoryDBIfUpdateIsFinished(void);
+void reap_update_task(void);
 
 int isUpdatingDB(void);