Improve TLSv1.3 server handling of no shared groups.

While we currently correctly handle the no-shared-group case, it currently
fails late when we try to create the key share. Improve detection and
handling so that we fail sooner and send an alert to the client when
processing client key shares.

While here rename preferred_group_found to shared_group_found - we look for
the client preferred group, but any group that we select will always be in
the client list (even if it's the last one).

Reported by the tlspuffin team.

ok tb@

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