sslutil: add a config knob to support TLS (default) or SSLv23 (bc) (issue4038) Prior to this change, we default to SSLv23, which is insecure because it allows use of SSLv2. Unfortunately, there's no constant for OpenSSL to let us use SSLv3 or TLS - we have to pick one or the other. We expose a knob to revert to pre-TLS SSL for the user that has an ancient server that lacks proper TLS support.