crypto: allow forcing SSLv2/v3 via secureProtocol

Force-enable SSLv2/v3 when `secureProtocol` is explicitly set
to `SSLv2_method` or `SSLv3_method`.

see discussion at #8551
This isn't 100% right, the secureProtocol should also handle the `undefined` case, but should happen below not when we're defining the default options, it should be set when `flags` is `undefined` and when `secureProtocol` isn't `SSLv23...`