This is CVE-2016-0701 for OpenSSL, reported by Antonio Sanso. It is a no-op for
us as we'd long removed SSL_OP_DH_SINGLE_USE and static DH cipher suites. (We
also do not parse or generate X9.42 DH parameters.)

However, we do still have the APIs which return RFC 5114 groups, so we should
perform the necessary checks in case later consumers reuse keys.

Unlike groups we generate, RFC 5114 groups do not use "safe primes" and have
many small subgroups. In those cases, the subprime q is available. Before using
a public key, ensure its order is q by checking y^q = 1 (mod p). (q is assumed
to be prime and the existing range checks ensure y is not 1.)

(Imported from upstream's 878e2c5b13 and
75374adf8a6ff69d6718952121875a491ed2cd29, but with some bugs fixed. See
RT4278.)

Change-Id: Ib18c3e84819002fa36a127ac12ca00ee33ea018a
Reviewed-on: https://boringssl-review.googlesource.com/7001
Reviewed-by: Adam Langley <agl@google.com>