[IZPACK-1045] - Do not ignore privileged option for console and automatic installations

This is to address http://jira.codehaus.org/browse/IZPACK-1045

1. On Window's machine always ask prompt to escalate permissions through the UAC prompt if the installer is marked to run as privileged user.
2. On Unix machines, always inform the user to run installer with administrative (root) privileges if the installer is marked to run as privileged user, and current user is not root (id of 0)