directory: moved code to update.c

The source directory.c mixes several libraries: directory object
management, database management and database update, resulting in a
1000+ line monster.  Move the whole database update code to update.c.

src/main.c

@@ -20,6 +20,7 @@
 #include "command.h"
 #include "playlist.h"
 #include "directory.h"
+#include "update.h"
 #include "player_thread.h"
 #include "listen.h"
 #include "conf.h"