Import certs for s/mime autoresolve from keyserver

* src/keycache.cpp (do_locate): Factor out S/MIME lookup.
(get_extern_smime_keys): New.
(get_local_smime_keys, get_most_valid_key_simple): Factored
out from do_locate.

This implements the feature that X509 Certificates are
automatically searched and imported from the keyserver if
they are not available locally or if all local certs
are expired / revoked etc.

• GnuPG-Bug-Id: T4174