Bug#183360: prebaseconfig: 'cancel' button do not cancel

[Petter Reinholdtsen <pere@hungry.com>]

Package: prebaseconfig
Version: 0.024

At the end of prebaseconfig, the user is told that the machine will
now reboot from hard drive.  When using DEBIAN_FRONTEND=slang, two
buttons appear, <next> and <cancel>.  When pressing <cancel>, I expect
the installation to stop, and not reboot the machine.  The current
implementation instead continues with the reboot, just as if I had
pressed <next>.

If the <cancel> button appear, it should work.