gpg: Update key origin info during import merge.

* g10/import.c (update_key_origin): New.
(merge_blocks): Add arg curtime.
(import_one): Pass curtime to merge_blocks.  Call update_key_origin.

We probably need to refine the rules on how this is done. But it is a
start.

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