Disable hung task timeout in installer and enter-chroot.

Large writes on the Samsung ARM Chromebook, especially to external devices
(SD card or USB stick) can cause a task to be stuck for longer than 120
seconds, which would cause a kernel panic (and an immediate reboot).

Instead of disabling the timeout altogether, we just make sure the kernel
does not panic by writing 0 to `/proc/sys/kernel/hung_task_panic` (this is
the default configuration of a vanilla kernel).

See http://code.google.com/p/chromium/issues/detail?id=260955 for details,
and why this may not be fixed upstream. Also fixes #319.