tty: Use separate tty_info::inherit member to track ttys being borrowed from a parent

This is easier for debugging, the former SID/PGID remains untouched
and printed out in logs.

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