tty: Restore controlling terminal once per session

There might be several same terminals opened (say
tty6 or whatever) which gonna look as separate
files but actually pointing to the same tty kernel
instance. Moreover if it's a controlling terminal
we will be trying to restore it as many times as
find non zero sid on a peer.

Instead lets do a simple trick first: choose a leader
from a terminal group and use it only for controlling
terminal restoration.

https://jira.sw.ru/browse/PSBM-40969



Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Reviewed-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>