Minimal isn't always best.
* sign.c (update_keysig_packet): Use the current time rather then a
modification of the original signature time. Make sure that this doesn't
cause a time warp.
* keygen.c (keygen_add_key_expire): Properly handle a key expiration date
in the past (use a duration of 0).
* keyedit.c (menu_expire): Use update_keysig_packet so any sig subpackets
are maintained during the update.
* build-packet.c (build_sig_subpkt): Mark sig expired or unexpired when
the sig expiration subpacket is added. (build_sig_subpkt_from_sig): Handle
making an expiration subpacket from a sig that has already expired (use a
duration of 0).
* packet.h, sign.c (update_keysig_packet), keyedit.c
(menu_set_primary_uid, menu_set_preferences): Add ability to issue 0x18
subkey binding sigs to update_keysig_packet and change all callers.