When building subpackets for a Signature, ensure a "reason for
revocation" subpacket is added if the Signature is a
SigTypeKeyRevocation.

See https://tools.ietf.org/html/rfc4880#section-5.2.3.23

Rather than using the *first* subkey binding signature encountered, use
the one with the most recent creation data, as per the recommendation from RFC
4880:

> An implementation that encounters multiple self-signatures on the
> same object may resolve the ambiguity in any way it sees fit, but it
> is RECOMMENDED that priority be given to the most recent self-
> signature.

This allows subkeys to approach expiry then be re-signed with a new
expiry.

openpgp: use latest subkey binding signature

rather than type alias.