WIP: SSL/TLS: Add OpenSSL support

I'm marking this as WIP because it's not finished yet. Missing stuff is listed below.

This has been paused since late December, but I'm continuing work now.

You can review this if you want now.

Missing features

• OCSP: due to its complexity, I'm developing it separately in branch wget2-openssl-ajuaristi-ocsp. Will merge onto here where it's ready.
• TCP Fast Open
• Integration with stats module

Approver's checklist:

• The author has submitted the FSF Copyright Assignment and is listed in AUTHORS
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent with existing code
• This feature/change has adequate documentation added (if appropriate)
• No obvious mistakes / misspelling in the code