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
Well that is the reason I changed the syntax here: https://github.com/SpriteLink/NIPAP/pull/458

Another parameter seems to be pretty weird. So should it look like this?
``nipap-passwd --test-user --username test --password test``

But then again if I add a user I specify the username with --add and when I delete one with --delete.

This makes me feel bad :(