gpg: Get the signature keyid from the issuer fpr.

* g10/parse-packet.c (parse_signature): Parse the ISSUER_FPR subpacket
and use that to get the keyid.

Because ADSKs are created w/o the issuer subpacket (despite that this
is still a v4 signature) we need to get the key id from the
issuer_fpr. This does not harm and we still fallback to the the
issuer. Note that for ease of future backporting we also take v5
fingerprints into account.