SSL: removed default DH parameters.

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).