cgroup: support --cgroup-root on dump too

The problem here is again caused by systemd :). Every process lives in some
child cgroup and not the root one, so we end up with a bunch of stuff like:

/init.scope
/system.slice/...
/user.slice/...

and nothing in the root cgroup. However, systemd opens
/sys/fs/cgroup/systemd, changes the perms, and keeps a fd around.
Unfortuantely, we don't track the perms on the "real root" cgroup here,
because nothing is at that level, so when we restore, our cgroup perms
changing code doesn't catch this perms change, and we get:

(00.361723)      1: Error (criu/files-reg.c:1487): File sys/fs/cgroup/systemd has bad mode 040755 (expect 040775)

To fix this, let's just support the --cgroup-root argument on dump too, and
rewrite the cgroup paths we dump accordingly.

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