restore: Set CLONE_PARENT iif pdeath_sig is present, v4

It's been discovered that on 3.11 we might fail on restore
if pass @CLONE_PARENT flag into clone() call due to kernel
limitations.

Because we're treating 3.11 as a base working kernel lets
do a trick instead

 - setup this flag iif pdeath_sig is present
 - if CLONE_NEWPID is passed warn a user about
   potential consequences.
 - because we need to carry the condition in attach_to_tasks
   call, introduce @root_as_sibling variable for this.

CC: Tycho Andersen <tycho.andersen@canonical.com>
CC: Pavel Emelyanov <xemul@parallels.com>
CC: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>