gpg: Fix key expiration and usage for keys created at the Epoch.

* g10/getkey.c (merge_selfsigs_main): Take a zero key creation time in
account.

Keys created at the Epoch have a creation time of 0; when figuring out
the latest signature with properties to apply to a key the usual
comparison A > B does not work if A is always 0. We now special case
this for the expiration and usage data.

• Co-authored-by: gniibe@fsij.org
• GnuPG-bug-id: T4670
• Signed-off-by: Werner Koch <wk@gnupg.org>