Only allow email queries if no key/directory servers are configured

If no key servers (OpenPGP) and no directory servers (S/MIME) are
configured then the server lookup only performs a WKD lookup. And for
WKD lookups we need an email address.

Add a hint that an email address has to be entered and validate that the
input looks (more or less) like an email address if only email queries
are possible.

(cherry picked from commit a49b3b66bd8b225a50d48ff2ee751f97f280c83d)

• GnuPG-bug-id: T6868