Add Next Protocol Negotiation functions for OpenSSL

Next Protocol Negotiation was added in OpenSSL 1.0.1. This pull request would expose the necessary functions for use in pyca/pyopenssl#79.

I've had some testing 'fun' with this, probably because I've got no experience with CFFI or with OpenSSL. I'm opening this PR to see whether your CI infrastructure likes it or not. If it doesn't, I'll try to chase down why.