directory: moved code to database.c

Taming the directory.c monster, part II: move the database management
stuff to database.  directory.c should only contain code which works
on directory objects.

src/update.c

@@ -18,6 +18,7 @@
  */
 
 #include "update.h"
+#include "database.h"
 #include "directory.h"
 #include "song.h"
 #include "log.h"