openpgp: use latest subkey binding signature
openpgp: use latest subkey binding signature

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.

This extends the recent commit 0e37d00 by @aviau and @FiloSottile.

Fixes golang/go#26468

Change-Id: I7f12706727373259c188bfee4254306ef9d4e935
GitHub-Last-Rev: 0da814166411a3c3334312576d3f7f8f2bba4ff4
GitHub-Pull-Request: golang/crypto#57
Reviewed-on: https://go-review.googlesource.com/135357
Reviewed-by: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>

golang/crypto,golang/crypto,golang/crypto