Change DH_generate_parameters back to order 2q subgroup

For for G=2 and 5 DH_generate_parameters will continue to generate
the order 2q subgroup for compatibility with previous versions.

For G=3 DH_generate_parameters generates an order q subgroup, but it
will not pass the check in DH_check with previous OpenSSL versions.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9820)