This breaks everything, because files.c can't have
a deal with this yet. But next patch will teach it.

v5: Use hard-coded 0, -1 and 1 for successful return,
error and "again" request.

v4: tty: merge new_fd assignment to receive_tty()
    and return this function result directly.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>