kbx: Fix keyid search for mixed v4/v5 case.

* kbx/keybox-search.c (blob_cmp_fpr_part):  Reworked.
(has_short_kid, has_long_kid): Simplify.

The old code was too complicated and did not cope correctly a blob
having a mix of v5 and v4 keys.

• Fixes-commit: 01329da8a778d3b0d121c83bfb61d99a39cccac4
• GnuPG-bug-id: T5888