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.
the OpenSSH public key format has no encapsulation boundaries. It's partially base64 encoded but the key format and comment are both ASCII as well. That would seem to fall outside the bounds of the actual PEM spec?