Expose the version octet from IssuerFingerprint subpacket.

[andrewgdotcom]

The first octet of an IssuerFingerprint subpacket indicates the version of the fingerprint (and therefore the key that made it), but the IssuerFingerprint field of the signature type omits the version octet. I think this version number should be exposed directly:

1. v5 and v6 fingerprints are the same length, so we cannot distinguish them by length
2. v5 keys are not limited to creating v5 signatures, so we cannot assume the key version is the same as the signature version

I plan to work around this in hockeypuck by noting that if an IssuerFingerprint has a length of 32 octets AND the signature version is not 6, then the fingerprint version must have been 5 - but this is kludgy and will not necessarily hold in future (GnuPG did experiment with allowing v5 keys to make "v6" signatures).

It would therefore be nice if there was an IssuerFingerprintVersion field in the signature type.