Use poll() instead of select()

Fixes docker#2278
because poll does not have select's builtin limit of 1024 descriptors

Signed-off-by: Jan Bieron <jbieron+github@gmail.com>

docker/utils/socket.py

@@ -28,7 +28,9 @@ def read(socket, n=4096):
     recoverable_errors = (errno.EINTR, errno.EDEADLK, errno.EWOULDBLOCK)
 
     if six.PY3 and not isinstance(socket, NpipeSocket):
-        select.select([socket], [], [])
+        poll = select.poll()
+        poll.register(socket)
+        poll.poll()
 
     try:
         if hasattr(socket, 'recv'):