Encrypts a previously loaded RSA private key and stores it into a variable in PKCS#8 format
rsa_save_privatekey_mem( privkey var, rsa var, passphrase )
Returns 1 on success, 0 on error.
privkey = The variable which will receive the encrypted private key
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.