There are places when we have to select which fd to ->open
and which to ->receive. To avoid deadlocks we sort them in
an ascending order on { pid, fd } pair.

Make this idea more formal by introducing an explicit function
doing this check and call it where appropriate (pipe.c master
selection is also simplified to fit new ... API).
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>