Support disabling SSL protocol versions

Given the [vulnerabilities in SSLv3](https://www.openssl.org/~bodo/ssl-poodle.pdf) it'd be good to support disabling it, using SSL_OP_NO_SSLv3 and similar.

Example usage:
```ocaml
Ssl.disable_protocols !Ocsigen_http_client.sslcontext [Ssl.SSLv23];
```