Relax private DNSKEY and RRSIG constraints

Relax private DNSKEY and RRSIG constraints

DNSKEY, KEY, RRSIG and SIG constraints have been relaxed to allow
empty key and signature material after the algorithm identifier for
PRIVATEOID and PRIVATEDNS. It is arguable whether this falls within
the expected use of these types as no key material is shared and
the signatures are ineffective but these are private algorithms and
they can be totally insecure.