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
Does it really need a short form?
You could potentially use -T but I really think it is enough with the long form since this is a command you typically don't run very often.