With this patch, the daemon can perform chrooted builds on Hurd, without
creating problems to other parts of the daemon that can't be supported.

So as Mark said, the cases are:

1. CONTAINER_ENABLED and CHROOT_ENABLED are both true.
In this case, the daemon works as expected, which is what happens in Linux now.

2. CONTAINER_ENABLED is false and CHROOT_ENABLED is true.
Here, things like namespaces cannot be supported, but we can still
perform chrooted builds.

3. CONTAINER_ENABLED and CHROOT_ENABLED are both false.
Here, the daemon is unusable on the system, as it should.