build: add NetBSD support to opensslconf.h

Simplify the BSD list by defining OPENSSL_BSD if using a matching
BSD platform.
Add NetBSD to the list and update documentation.

PR-URL: https://github.com/nodejs/node/pull/14313


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>