Make EVP_PKEY_CTX_[get|set]_group_name work for ECX too

Make EVP_PKEY_CTX_[get|set]_group_name work for ECX too

The previous commits made EVP_PKEY_CTX_[get|set]_group_name work for
EC and DH keys. We now extend this to ECX. Even though that keys with
these key types only have one group we still allow it to be explicitly
set so that we have only one codepath for all keys. Setting the group
name for these types of keys is optional, but if you do so it must have
the correct name.

Additionally we enable parameter generation for these keys. Parameters
aren't actually needed for this key type, but for the same reasons as
above (to ensure a single codepath for users of these algorithms) we
enable it anyway.

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11914)