Issue #12328: Fix multiprocessing's use of overlapped I/O on Windows.

Also, add a multiprocessing.connection.wait(rlist, timeout=None) function
for polling multiple objects at once.  Patch by sbt.

Complete changelist from sbt's patch:

* Adds a wait(rlist, timeout=None) function for polling multiple
  objects at once.  On Unix this is just a wrapper for
  select(rlist, [], [], timeout=None).

* Removes use of the SentinelReady exception and the sentinels argument
  to certain methods.  concurrent.futures.process has been changed to
  use wait() instead of SentinelReady.

* Fixes bugs concerning PipeConnection.poll() and messages of zero
  length.

* Fixes PipeListener.accept() to call ConnectNamedPipe() with
  overlapped=True.

* Fixes Queue.empty() and SimpleQueue.empty() so that they are
  threadsafe on Windows.

* Now PipeConnection.poll() and wait() will not modify the pipe except
  possibly by consuming a zero length message.  (Previously poll()
  could consume a partial message.)

* All of multiprocesing's pipe related blocking functions/methods are
  now interruptible by SIGINT on Windows.

Lib/concurrent/futures/process.py

@@ -50,7 +50,8 @@ import os
 from concurrent.futures import _base
 import queue
 import multiprocessing
-from multiprocessing.queues import SimpleQueue, SentinelReady, Full
+from multiprocessing.queues import SimpleQueue, Full
+from multiprocessing.connection import wait
 import threading
 import weakref
 
@@ -212,6 +213,8 @@ def _queue_management_worker(executor_reference,
         for p in processes.values():
             p.join()
 
+    reader = result_queue._reader
+
     while True:
         _add_call_item_to_queue(pending_work_items,
                                 work_ids_queue,
@@ -219,9 +222,10 @@ def _queue_management_worker(executor_reference,
 
         sentinels = [p.sentinel for p in processes.values()]
         assert sentinels
-        try:
-            result_item = result_queue.get(sentinels=sentinels)
-        except SentinelReady:
+        ready = wait([reader] + sentinels)
+        if reader in ready:
+            result_item = reader.recv()
+        else:
             # Mark the process pool broken so that submits fail right now.
             executor = executor_reference()
             if executor is not None: