TTY masters and slaves have no post_open stage,
so these two blocks may safely have their stages merged together.

The third is eventpoll, but two above do not depend
on it (their .post_open do not depend on eventpoll .open).
Unix sockets would have been, but this isn't implemented yet.

So, we may safely execute all stages for different
file types separatelly.

travis-ci: success for Rework file opening scheme to make it asynchronous (rev5)
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>