tty: Parse slave peer index from fd_parms

We will use this path in reg-files engine anyway
so simply switch to this ability now.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>