win32: revise poll(2) for use in nc

Upstream commit 5b3b468ec (nc: use poll() instead of select())
changed `nc` to use poll(2) instead of select(2), obviously.

In busybox-w32 select(2) had already been hacked to support `nc`.
To avoid hacking poll(2) too the upstream change was reverted
(c2002eae3).  Later `nc` was altered to include the code for both
poll and select (3c85cc0c4).

Make the changes necessary for poll(2) to work with `nc` on Windows.
These are all in the function windows_compute_revents().

  Treat a character file that isn't a console as a normal file.
  Without this `nc 127.0.0.1 1234 </dev/null` doesn't work.

  Return 0 instead of POLLHUP if GetNumberOfConsoleInputEvents()
  indicates no events are available.  Without this communication
  between two instances of `nc` which are both using keyboard
  input isn't as asynchronous as one would like.

  Only process key press events:  key releases are ignored.  Without
  this `nc 127.0.0.1 1234` won't receive anything from the server
  until the local user presses a key.

  In the default case, which now includes disk files and character
  files, detect polling for reads as well as writes.  Without this
  `nc 127.0.0.1 1234 <local_file` doesn't work.

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