Adds TLSv1.3 detection & support to Nginx

I'm not sure yet if it is enough to check for the OpenSSL version or if it is also necessary to check for the running Kernel version to provide TLS 1.3. I think I've read somewhere that the Kernel has to be at least version 4.13.

Do we need to check for the correct Nginx version too? I actually tend to add a check for that too. Opinions?

edit
Gonna rework a few things because I doubt that the current implementation is "safe" enough. Checking for the OpenSSL version alone won't be sufficient.