tty: Squash tty_get_sid, tty_get_pgrp into tty_get_sid_pgrp

This patch squashes tty_get_sid, tty_get_pgrp into
one tty_get_sid_pgrp helper (which allows to detect
if dumping terminal is hanging up, the real use of
this ability will be addressed in next patch).

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>