Fixes Broken ED25519 OpenSSH Private Key Exports

[MCterra10]

Resolves a few issues in the binary format of exported OpenSSH Private Key files, namely:

• The nonce was generated as a single 64bit random string instead of a 32bit string repeated twice.
• Null comment was omitted
• Padding was omitted

After implementing these changes and building XCA, I was able to confirm ED25519 keys are now working with OpenSSH:

fixes #658