gpg: Implement searching keys via keygrip.

* kbx/keybox-defs.h (struct _keybox_openpgp_key_info): Add field grip.
* kbx/keybox-openpgp.c (struct keyparm_s): New.
(keygrip_from_keyparm): New.
(parse_key): Compute keygrip.
* kbx/keybox-search.c (blob_openpgp_has_grip): New.
(has_keygrip): Call it.

This has been marked for too long as not yet working. However, it is
a pretty useful feature and will come pretty handy when looking for
all keys matching one keygrip.

Can be optimized a lot by storing the keygrip in the meta data. This
will be done along with the upgrade of KBX for v5 fingerprints.

• Signed-off-by: Werner Koch <wk@gnupg.org>