kernel - Implement POLLHUP for pipes and filesystem fifos (3)

kernel - Implement POLLHUP for pipes and filesystem fifos (3)

* Add an internal NOTE_HUPONLY flag to allow the poll() system call
  to tell the kevent system that EVFILT_READ should only trigger on
  a HUP and not trigger on read-data-present.

* Linux does not trigger POLLHUP on a half-closed socket, make
  DFly have the same behavior.  POLLHUP is only triggered on a fully-closed
  socket.

* Fix bug where data-present on the pipe, socket, or fifo would trigger an
  EVFILT_READ event when only a HUP is being requested.  This caused our
  poll() implementation to complain about spurious events (which then
  results in incorrect operation).