Use accessors for DSA and RSA objects

Use accessors for DSA and RSA objects

In the upcoming LibreSSL version 3.5, the DSA and RSA structs will
become opaque, so we should use the OpenSSL 1.1 setters and getters
to fix the build.  The relevant API has been available since
LibreSSL 2.7. The last LibreSSL version (2.6) not having these
accessors was EOL late 2018, so no users should be affected by this
change.