tty: open all terminals with O_NOCTTY

Otherwise the kernel can set it as a control terminal for the currect
session and CRIU will not be able to set it as a control terminal for
the target process.

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