gpg: Speed up key listing in Tofu mode.

* g10/tofu.c (get_trust): Add arg PK.  Uses this instead of a an extra
lookup of the public key by fingerrpint.
(tofu_register): Pass PK to get_trust.
(tofu_get_validity): Ditto.
*g10/tofu.c (tofu_register): Remove unused FINGERPRINT_PP.

With my test keybox I see a speedup of 10 times (33s to 3.1s). The
reason for this was the extra key lookup which I hacked in at some
point to make the extraction of a keyid correct also for non v4 keys.
However our caller already has the public key and thus can easily pass
it to get_trust along with the fingerprint.

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