SSL: removed default DH parameters. Using the same DH parameters on multiple servers is believed to be subject to precomputation attacks, see http://weakdh.org/. Additionally, 1024 bits are not enough in the modern world as well. Let users provide their own DH parameters with the ssl_dhparam directive if they want to use EDH ciphers. Note that SSL_CTX_set_dh_auto() as provided by OpenSSL 1.1.0 uses fixed DH parameters from RFC 5114 and RFC 3526, and therefore subject to the same precomputation attacks. We avoid using it as well. This change also fixes compilation with OpenSSL 1.1.0-pre5 (aka Beta 2), as OpenSSL developers changed their policy after releasing Beta 1 and broke API once again by making the DH struct opaque (see ticket #860).