_NetBSD-pkgdb: Add migration script

This script can check if all is consistent, fix consistency issues
(without migrating), or actually migrate.

I am running out of systems to test migration on.  This script should
be, for now, viewed with suspicion, and run in fix or migrate mode
only on systems with good backups, or for which you genuinely don't
care if you have to start over with pkgsrc.

(gdt)