gpg: With --auto-key-retrieve prefer WKD over keyservers.

* g10/mainproc.c (check_sig_and_print): Print a hint on how to make
use of the preferred keyserver.  Remove keyserver lookup just by the
keyid.  Try a WKD lookup before a keyserver lookup.

The use of the the keyid for lookups does not make much sense anymore
since for quite some time we do have the fingerprint as part of the
signature.

• GnuPG-bug-id: T4595
• Signed-off-by: Werner Koch <wk@gnupg.org>