crypto: use RSA and DH accessors

Parts of this were cherry-picked from PR #8491. Note that this only
works with OpenSSL 1.0.2 or 1.1.0g or later. 1.1.0g is, as of writing,
not yet released, but the fix is on the branch. See
https://github.com/openssl/openssl/pull/4384.

PR-URL: https://github.com/nodejs/node/pull/16130


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Rod Vagg <rod@vagg.org>