Store groups as uint16_t

Store groups as uint16_t

Instead of storing supported groups in on-the-wire format store
them as parsed uint16_t values. This simplifies handling of groups
as the values can be directly used instead of being converted.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4406)