gpg: Read the UBID from the keybox and detect wrong blob type.

* g10/keydb-private.h (struct keydb_handle_s): Add fields for UBID.
* g10/call-keyboxd.c (search_status_cb): New.
(keydb_search): Set new UBID fields.

The UBID is not yet used but may come handy later.

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