net/samba422: Fix location of ldb files

• Use SAMBA_LDB_MODULESDIR consistently
• Move all AD_DC-related plist entries to pkg-plist.ad_dc

This patch fixes a simple "make package" build with default options.

PR: 291541
Reported by: O. Hartmann <ohartmann@walstatt.org>
Sponsored by: Klara, Inc.