openzfs: attach pam_zfs_key to build

This PAM module allows unlocking encrypted user home datasets when
logging in (and changing passphrase when changing the account password),
see https://github.com/openzfs/zfs/pull/9903

Also supposed to unload the key when the last session for the user is
done, but there are EBUSY issues:
https://github.com/openzfs/zfs/issues/11222#issuecomment-731897858

Submitted by: Greg V <greg_unrelenting.technology>
Reviewed by: mm
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D28018