crypto: fix openssl.cnf FIPS handling & testing

* Add documentation for `--openssl-conf=file`.
* Fix openssl.cnf loading and OpenSSL init ordering
* Fix FIPS tests so `OPENSSL_CONF` is not longer usable but
  `--openssl-conf` is

PR-URL: https://github.com/nodejs/node-private/pull/82


Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>