files: Implement {set,clear,wait}_fds_event()

The idea is symilar to kernel's wake_up() and wait_event().
One task needs some event. It checks the event has not
happened yet (fle hasn't received, unix peer hasn't bound, etc)
and calls get_fds_event(). Other task makes the event
(sends a fle, binds the peer to a name, etc) and calls set_fds_event().

So, while there is no an event, the first task is sleeping,
and the second wakes it up later:

Task A:      clear_fds_event();
             if (!socket_bound)
                     wait_fds_event(); /* sleep */

Task B:      bind_socket();
             set_fds_event();         /* wake up */

For the details of using see next patches.

v5: Use bit operations.
    Split clear_fds_event from wait function.

v2: Do not wait for foreign transport sock is ready,
as it's guarantied by we create it before CR_STATE_FORKING.

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>