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/command.c

@@ -21,6 +21,7 @@
 #include "playlist.h"
 #include "ls.h"
 #include "directory.h"
+#include "update.h"
 #include "volume.h"
 #include "stats.h"
 #include "list.h"