Instead of relying on the old ptrace loop, we should instead put all the
tasks in the container into the freezer. This will stop them all at the
same time, preventing fork bombs from causing criu to infinite loop (and is
also simply a lot faster).

Note that this uses --freeze-cgroup which isn't in criu 1.7, so it should
only go into master.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>