MSFT makes the `q` field of `DomainParameters` OPTIONAL even though it's
actually required. We currently validate DH groups not by validating
that p is a Sophie Germain prime but by checking the proposed group
against a compiled-in list and against a krb5.moduli file, therefore we
don't need q. Besides, for Oakley groups, because p is a Sophie Germain
prime, we'd have q=p/j and j=2, so we can always compute q as needed
(and MIT Kerberos does).
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