Per https://www.openssl.org/docs/crypto/pem.html, specifying to encrypt
a private key with DES *without* providing a passphrase causes OpenSSL
to prompt for the passphrase to use on the console -- which is
problematic for GUI applications which generally lack a console.
This modifies the behavior of RSAKeyImpl::save() so that DES encryption
of private keys will not be attempted unless the passphrase argument is
non-empty. This will also suppress OpenSSL's prompting for that
passphrase on the console.
Also added a case to the test suite.
the hash before signing. That way we can then support RSA signatures
with any hash supported by OpenSSL, not only MD5 and SHA1. It was also
required a new method on DigestEngine to obtain the NID (numeric ID) of
the algorithm, to be used on the OpenSSL RSA_sign call.
