Support for traditional OpenSSL and PKCS8 RSA private key serialization

This temporarily uses the backend to do it until the alternate approach is ready. DSA/ECDSA and public key support will follow if this is deemed worthy.
Passing something which is neither `BestAvailable` nor `NoEncryption` seems to be silently ignored. I think this and the above `if` statements should be joined, and have a proper `else` which raises an error.