Add verify to nipap-passwd

Add new feature to nipap-passwd to verify if a username and password is correct. The tool will show if the user exists and if the password is correct.

For request https://github.com/SpriteLink/NIPAP/issues/467
I think --verify is a tad ambiguous. Are we verifying integrity of the database? Testing authentication credentials of a user?

Perhaps --test-credentials ? --test-user?