don't use select.POLL* constants

don't use select.POLL* constants

Python doesn't have select.POLL* constants on some architectures
(e.g. MacOSX). This code needs to define the constants for itself. It
uses select.POLL* constants only internally (doesn't pass them
outside). So there is no harm even if the definition would conflict
with Python's those.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ben Pfaff <blp@nicira.com>