gpg: Lookup key for merging/inserting only beu primary key.

* g10/getkey.c (get_keyblock_byfpr_fast): Add arg primary_only and
implement.
* g10/import.c (import_one_real): Simplify filling the fpr buffer with
zeroes.
(import_one_real): Find key only by primary fingerprint.

This should have been done early: When looking up the original
keyblock we want to update, we need to lookup it up only using the
primary key. This avoids to find a key which has the primary key also
has a subkey.

• GnuPG-bug-id: T7527