registrar: broken /etc/passwd and etc/shadow files are created if string fields contains colon character

[X512]

This is hrev54723.

Steps to reproduce:

1. Run useradd "user:::".
2. user::::x:1005:100::/boot/home:/bin/sh invalid entry will be created to /etc/passwd.
3. Run userdel "user:::" before reboot to avoid system get messed up.

registrar should perform input data check or escaping.

Related code: ​AuthenticationManager.cpp:1006

I created GUI utility to mange users and groups that can be also useful for investigation: ​Users.