gpg-interface.c: support getting key fingerprint via %GF format

gpg-interface.c: support getting key fingerprint via %GF format

Support processing VALIDSIG status that provides additional information
for valid signatures.  Use this information to propagate signing key
fingerprint and expose it via %GF pretty format.  This format can be
used to build safer key verification systems that verify the key via
complete fingerprint rather than short/long identifier provided by %GK.

Signed-off-by: Michał Górny <mgorny@gentoo.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>