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
Yeah, my comment in #458 sounded overly negative. It was probably because it wasn't just a swap of optparse for argparse but also included syntax changes.

With such a change, the debian scripts and various documentation needs to be updated.