gpg: Refresh expired keys originating from the WKD.

* g10/getkey.c (getkey_ctx_s): New field found_via_akl.
(get_pubkey_byname): Set it.
(only_expired_enc_subkeys): New.
(get_best_pubkey_byname): Add support to refresh expired keys from the
WKD.

A little drawback of that code is that if the WKD has no update for an
expired key each access of the key will trigger a WKD lookup (unless
cached by the dirmngr). To avoid this we need to record the last time
we have checked for an update but that would in turn require that we
update the keyring for each check. We defer this until we have a
better key database which allows for fast updates of meta data.

Testing the code is currently a bit cumbersome because it requires to
update a key in the WKD several times. Eventually we we need a
network emulation layer to provide sample data for the regression
tests.

• GnuPG-bug-id: T2917
• Signed-off-by: Werner Koch <wk@gnupg.org>