gpg: Update --trusted-key to accept fingerprint as well as long key id.

* g10/trustdb.c (tdb_register_trusted_key): accept fingerprint as well
as long key ID.
* doc/gpg.texi: document that --trusted-key can accept a fingerprint.

Fixed uses or return and kept the old string to avoid breaking
translations.

• GnuPG-bug-id: T4855
• Signed-off-by: Werner Koch <wk@gnupg.org>