gpg: Fix trustdb for v5key.

* g10/keydb.h (fpr20_from_pk): New.
* g10/keyid.c (fpr20_from_pk): New.
* g10/tdbio.c (tdbio_search_trust_byfpr): Use fpr20_from_pk.
* g10/trustdb.c (keyid_from_fpr20): New.
(verify_own_keys): Use keyid_from_fpr20.
(tdb_update_ownertrust): Use fpr20_from_pk.
(update_min_ownertrust): Likewise.
(update_validity): Likewise.

For the compatibility of existing implementation, we keep the format
of trustdb untouched. The format of trustdb uses 20-byte fingerprint
for the trust record entry. To handle both of v4key (with 20-byte
fingerprint) and v5 key (with 32-byte fingerprint), we introduce FPR20
fingerprint, internally. For v4key, FPR20 is as same as v4
fingerprint. For v5key, FPR20 is constructed from v5key fingerprint.

• GnuPG-bug-id: T5000
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>