tls: make server not use DHE in less than 1024bits

DHE key lengths less than 1024bits is already weaken as pointed out in
https://weakdh.org/ . 1024bits will not be safe in near future. We
will extend this up to 2048bits somedays later.

PR-URL: https://github.com/nodejs/io.js/pull/1739


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Fedor Indutny <fedor@indutny.com>