Prevent small subgroup attacks on DH/DHE

Historically OpenSSL only ever generated DH parameters based on "safe"
primes. More recently (in version 1.0.2) support was provided for
generating X9.42 style parameter files such as those required for RFC
5114 support. The primes used in such files may not be "safe". Where an
application is using DH configured with parameters based on primes that
are not "safe" then an attacker could use this fact to find a peer's
private DH exponent. This attack requires that the attacker complete
multiple handshakes in which the peer uses the same DH exponent.

A simple mitigation is to ensure that y^q (mod p) == 1

CVE-2016-0701

Issue reported by Antonio Sanso.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>