issue #249: restore duplex behaviour for epoll

With epoll() there is only one kernel-side object per file descriptor,
which is why _control() is such a pain. Since we merge receive/transmit
watching into that single object, we must always test the mask for both
conditions when reading results.

Kqueue isn't/doesn't appear to be like this. The identity of a Kqueue
event is keyed on (fd, filter), and we register a separate event for
both transmit and receive, so the 'elif' in KqueuePoller.poll() does not
appear to need to change.

Previously, a FD marked for read+write would not indicate writeability
until it was no longer readable.

mitogen/parent.py

@@ -647,7 +647,7 @@ class EpollPoller(Poller):
                 # Events can still be read for an already-discarded fd.
                 mitogen.core._vv and IOLOG.debug('%r: POLLIN: %r', self, fd)
                 yield self._rfds[fd]
-            elif event & select.EPOLLOUT and fd in self._wfds:
+            if event & select.EPOLLOUT and fd in self._wfds:
                 mitogen.core._vv and IOLOG.debug('%r: POLLOUT: %r', self, fd)
                 yield self._wfds[fd]