tty: allocate a separate tty info id for each external tty

Otherwise all external tty-s will share the same tty info,
what is obviously wrong.

v2: allocate a separate tty index range for externl terminals

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrei Vagin <avagin@gmail.com>