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 was thinking about returning 0 if it verified successfully (without exit code being influenced by the result). Do you think it should return 2 if the user does not exist or the password is wrong?