Provide GRUB2 support in KDM.

Allows for rebooting once in one of the entries of GRUB2 and then
automatically reverts to the default one.

Makes use of grub-reboot and therefore requires 'GRUB_DEFAULT=saved'
to be set in /etc/default/grub (so that /boot/grub/grub.cfg contains
'set default="${saved_entry}"'). If not set, no reboot list appears.

Performs proper GRUB un-escaping, as described in GRUB's manual: