gpg: Implement skip function for keyboxd to fix a validation bug.

* kbx/keybox-openpgp.c (_keybox_parse_openpgp): Add flag only_primary.
Change all callers.
(kbx_get_first_opgp_keyid): New.
* kbx/keybox-search-desc.h: Add prototype for this function.
* g10/call-keyboxd.c (keydb_search): Implement the skip function.

Instead of usig a skip function we may eventually want to move this to
keyboxd. For example we could tell keyboxd on a per session base a
list of keys it should not return at all.

• GnuPG-bug-id: T7983