Always generate DH keys for ephemeral DH cipher suites

Modified version of the commit ffaef3f1 in the master branch by Stephen
Henson. This makes the SSL_OP_SINGLE_DH_USE option a no-op and always
generates a new DH key for every handshake regardless.

CVE-2016-0701 (fix part 2 or 2)

Issue reported by Antonio Sanso

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