Add a patch to 'halt' that will shutdown all the processes within contexts

Add a patch to 'halt' that will shutdown all the processes within contexts
before everything else.

This is necessary because without this, a node would try to shutdown but hang
indefinitely with error messages like:

    umount: /vservers/<slicename>/proc : device is busy
    umount2: Device or resource busy

and finally:

    INIT : no more processes left in this runlevel

But not actually reboot.  This patch has worked in an alpha env where the
original halt script had failed.