gpg: Change the ADSK key binding time to the current time.

* g10/keyedit.c (append_adsk_to_key): Add arg sigtimestamp and pass
that as signing time to make_keysig_packet.
* g10/keygen.c (append_all_default_adsks): Likewise.
(do_generate_keypair): Likewise.

For not anymore known reasons or maybe because I mixed up creation and
binding time the creation date of the ADSK was also used as the time
for the key binding signature. This works but is a bit surprising.
Now we use either the current time or, on key creation, the time
creation time of the primary key as we also do with the other
self-signatures.

• GnuPG-bug-id: T6882