tty: Mark collected tty in @tty_active_pairs bitmap

Upon reception of tty we need to mark it in @tty_active_pairs
bitmam thus we will be able to verify if

 - all active ttys obtained are paired
 - if not paired then only one external tty is allowed

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