Reject unimplemented critical PGP packets as per RFC-4880

[pmatilai]

Reject unimplemented critical PGP packets as per RFC-4880

    Bit 7 of the subpacket type is the "critical" bit.  If set, it
    denotes that the subpacket is one that is critical for the evaluator
    of the signature to recognize.  If a subpacket is encountered that is
    marked critical but is unknown to the evaluating software, the
    evaluator SHOULD consider the signature to be in error.

We only implement creation time and issuer keyid, everything else is
unimplemented and should be flagged as an error if critical as per above.

Initial patch by Demi Marie Obenour.

[pmatilai]

Another piece based on PGP parser PR's by @DemiMarie but taking care to really track the implemented status, and keep debug printing intact.