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.
I'm concerned about these two options. Specifically, though these only include the cipher name, they also implicitly include a KDF, which differs depending on where you stick them. It's not clear to me this is the right way to represent that. What do you think about just having the BestAvailable at first?