kernel - make POLLHUP conform better to OpenGroup

kernel - make POLLHUP conform better to OpenGroup

* The poll() backend now only evaluates EV_EOF for EVFILT_WRITE, and
  generates a POLLHUP which is properly mutually exclusive with
  POLLWRNORM and POLLOUT.

  We do this even for a half-closed (write-side-closed) socket.

* We no longer set POLLHUP for POLLRD* or POLLIN, instead relying on
  the userland doing the read() and checking the 0 result (which
  userland has always done historically).

  EV_EOF for EVFILT_READ cannot be used to set POLLHUP for POLLRD* or
  POLLIN because it could indicate a half-closed connection
  (read-side-closed) where writing is still allowed, which is a more
  common situation on sockets.

  We would have to add another EV_ flag to set POLLHUP for POLLRD* or
  POLLIN to detect the fully disconnected state.  The OpenGroup standard
  does allow POLLHUP to be mixed with POLLRD* or POLLIN but for now we
  just don't set it at all in that case.

  Only a POLLOUT/POLLWRNORM flag request can cause POLLHUP to be set
  for now.