Save a private key string to an (unencrypted) PKCS-8 private key info file.
public static int SavePrivateKeyInfo( string outputFile, string privateKey, Rsa.Format format )
Public Shared Function SavePrivateKeyInfo ( _ outputFile As String, _ privateKey As String, _ format As Rsa.Format _ ) As Integer
VB6/C equivalent: RSA_SavePrivateKeyInfo