When been playing wich checkpoint/restore of container I found
that we can't reuse existing controller if they were pre-created.
For example currently in PCS7 we're bindmount cgroups which belong
to a container in a form of

 /sys/fs/cgroup/<controller>/<container> ==> /sys/fs/cgroup/<controller>

so that CRIU dumps such configuration fine but on restore
it recreates controllers from the scratch which we would
like to bindmount them and ask CRIU to restore subcgroups
and their parameters.

So I extended --manage-cgroups option to take <mode> arguments.
Detailed description in docs.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>