Move the pkgdb compatibility error to a NetBSD-specific file to avoid
breakage for users who have other package managers that use /var/db/pkg
(Reported by Frederic Cambus on FreeBSD, OpenBSD)

Adjust warning: specifying PKGDB_DIR in mk.conf should be sufficient
for building packages for the compatibility pkgdb location.

This is still an error message, because:
- While we can handle the existing references of PKGDB_DIR, new ones
might be created.
- It is a minor inconvenience to people building packages from source.

(maya)