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
Sounds good to me.
0 should always be used for success. We already use 1 for different types of errors and you recently implemented 2 for saying "database not up to date" which isn't an error but a negative response. Since this is also a type of negative response, it makes sense to use 2.