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.
Yes, neither the OpenSSH public key format nor RFC 4716 are standard PEM. While RFC 4716 looks very similar to PEM, it uses four hyphens and a space on the delimiter lines, rather than 5 hyphens. If I remember right, there were also some other small differences, like no blank line between header lines and the BASE64-encoded data. So, if there's any plan to support those, it would be better to just have a dump() function and treat PEM & DER as part of the format argument along with other options, rather than having dump_pem() and dump_der().