openpgp: Decode the issuer-fingerprint signature subpacket.

* guix/openpgp.scm (SUBPACKET-ISSUER-FINGERPRINT): New variable.
(openpgp-signature-issuer-fingerprint): New procedure.
(key-id-matches-fingerprint?): New procedure.
(get-signature): Look for the 'issuer and 'issuer-fingerprint
subpackets.  Ensure the issuer key ID matches the fingerprint when both
are available.
(parse-subpackets): Handle SUBPACKET-ISSUER-FINGERPRINT.
* tests/openpgp.scm (%rsa-key-fingerprint)
(%dsa-key-fingerprint, %ed25519-key-fingerprint): New variables.
* tests/openpgp.scm ("get-openpgp-detached-signature/ascii"): Check the
result of 'openpgp-signature-issuer-fingerprint'.