tty: Mark collected tty in @tty_active_pairs bitmap
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 <7a1ea01eee6961eb1e372e3508c2670446d086f4@openvz.org>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

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