The easiest way to let normal users shutdown or reboot the
computer from the wdm login window, is to add the users'
login name to the /etc/group file under the group 'shutdown'.

An example line is shown below, placing 'root', 'user1' and
'user2' in the 'shutdown' group.
# in /etc/group file:
shutdown::102:root,user1,user2

Then, you need to run visudo and add the following lines
to the /etc/sudoers file.
# in /etc/sudoers file:
%shutdown	ALL=NOPASSWD:/sbin/poweroff,/sbin/reboot