gpg-verify-result: canonicalise key when looking up signatures

gpg-verify-result: canonicalise key when looking up signatures

Use gpgme_get_key to find the primary key for the key we are
looking for, and the primary key for each signature, and
compare these when looking up signatures.

The primary key is the first in the list of subkeys, which is
the normal key ID people use when referring to a GPG key as an
identity.

If the key has a signing subkey, signature->fpr will not match
the provided key_id, so looking up both keys and comparing the
primary key fingerprints ensures they are both canonicalised.

https://github.com/ostreedev/ostree/issues/608

Closes: #1036
Approved by: cgwalters