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>