Functions to restrict accepted protocols and honor server cipher preferences

The easiest workaround for the POODLE vulnerability is to disable SSLv3 support.
Refer to https://www.openssl.org/~bodo/ssl-poodle.pdf
This PR adds a function allowing to do so.

It also includes a function to honor the cipher preference from the server (i.e., allowing ocaml-ssl clients to do something equivalent to `SSLHonorCipherOrder true` in Apache).