key_load_private() ignores the passphrase argument if the private key
is unencrypted. This defeats the nullok check, because it means a
non-null passphrase will successfully unlock the key.

To address this, try at first to load the key without a passphrase.
If this succeeds and the user provided a non-empty passphrase *or*
nullok is false, reject the key.

MFC after: 1 week
Noticed by: Guy Helmer <guy.helmer@palisadesystems.com>