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/command.c

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