opts: add --manage-cgroups option

criu managed cgroups is now an opt-in thing, so by default criu does not manage
(i.e. dump or restore) cgroups. This allows users to use the previous behavior.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>