Encrypts and writes a previously loaded RSA private key to a file in PKCS#8 format
rsa_save_privatekey( privkey_file, rsa var, passphrase )
Returns 1 on success, 0 on error.
privkey_file = The name of the file in which the private key is to be stored
rsa = The internal reference to the RSA private key to be saved
passphrase = The passphrase used to encrypt the private key
The private key is encrypted using triple-DES in EDE mode with cipher block chaining (DES-EDE3-CBC) using the provided passphrase and stored in in PKCS#8 EncryptedPrivateKeyInfo format using PKCS#5 v2.0 password based encryption algorithms.