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>
tty: open tty-s with O_NOCTTY

When we open tty, we don't want to set it as controlling terminal.

[xemul: We do it in all the other places, this one is forgotten.
 The "controlling tty" feature is setup explicitly later with
 the ioctl (TIOCSCTTY) call. ]

This bug was caught by pty04. Where we get unexpected SIGCONT,
which is sent after closing a controlling terminal.

./pty04 --pidfile=pty04.pid --outfile=pty04.out
Dump 9578
Restore
Test: zdtm/live/static/pty04, Result: FAIL
==================================== ERROR ====================================
Test: zdtm/live/static/pty04, Namespace:
Dump log   : /home/jenkins/workspace/Rpi-CRIU/test/dump/static/pty04/9578/1/dump.log
--------------------------------- grep Error ---------------------------------
------------------------------------- END -------------------------------------
Restore log: /home/jenkins/workspace/Rpi-CRIU/test/dump/static/pty04/9578/1/restore.log
--------------------------------- grep Error ---------------------------------
(00.083420) Error (cr-restore.c:1092): 9578 killed by signal 0
(00.083708) Error (cr-restore.c:1713): Restoring FAILED.
------------------------------------- END -------------------------------------
================================= ERROR OVER =================================

Reported-by: Mr Jenkins
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>