pstree: Allow to dump and restore session non-leaders if --shell-job passed

If --shell-job passed we allow to dump and restpre session non-leaders.

Note that in sake of tty restore (which will be addressed in
further patches) we do inherit process group for root task.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>