Use pivot_root instead of chroot

The first patch is a straightforward change to fully detach from the root mount namespace.  The second is a more radical reworking of the filesystem arrangement.  It has two major benefits:

 * chroot is no longer used.  That means that standard chroot-breaking techniques won't apply.
 * The supervisor retains a bona fide capability to access the grain directory, rather than messing living in a different root.