sysvinit: allow users in shutdown group to perform halt/reboot
For this to happen:
* 'shutdown' group has been created;
* changed ownership group for /sbin/halt and /sbin/shutdown to 'shutdown';
* deny execution rights to other users except 'root' and those belonging
to 'shutdown' group;
* set setuid bit to both apps;
So, basically, in order for a normal user to be able to shutdown/reboot
the machine, it must be a member of 'shutdown' group.
Other changes:
* fixed identation for 2 lines that used spaces instead of tabs;
