Adds the requirement from RFC 8446 section 4.2.7:

   If the server has a group it prefers to the ones in the "key_share"
   extension but is still willing to accept the ClientHello, it SHOULD
   send "supported_groups" to update the client's view of its
   preferences; this extension SHOULD contain all groups the server
   supports, regardless of whether they are currently supported by the
   client.

Replaces current verification with a more specific one to comply with
RFC 8446 section 4.2.8:

   If using (EC)DHE key establishment and a HelloRetryRequest
   containing a "key_share" extension was received by the client, the
   client MUST verify that the selected NamedGroup in the ServerHello
   is the same as that in the HelloRetryRequest.  If this check fails,
   the client MUST abort the handshake with an "illegal_parameter"
   alert.