Support for CKC_OPENPGP, public key blobs to use SoftHSM under OpenPGP

This introduces CKC_OPENPGP as a case of CKC_VENDOR_DEFINED, with an unlikely-to-clash value that other vendors should be able to replicate on their HSMs.  The spec is close to CKC_X_509 but accurate details can be found on http://openfortress.nl/doc/spec/pgp-in-pkcs11/

Note 1.
I did not add test code, but I've used it successfully on my own system.  Let me know if you require test code, and what it should do -- and where to put it.

Note 2.
I did not find code to test which attributes could change when, so I did not modify any of that.  Please point me to such code if I missed it; the details are slightly different from CKC_X_509 as per the specification.  Notably, PGP public key blobs change over their life whereas X.509 certificates are static.