crypto,tls: remove SSLv2 support

Remove support for SSLv2 because of DROWN (CVE-2016-0800).

Use of the `--enable-ssl2` flag is now an error; node will print an
error message and exit.

PR-URL: https://github.com/nodejs/node/pull/5536


Reviewed-By: Rod Vagg <rod@vagg.org>