win32: revise poll(2) for use in nc

Upstream commit 5b3b468 (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
(c2002ea).  Later `nc` was altered to include the code for both
poll and select (3c85cc0).

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.

win32/poll.c

@@ -219,18 +219,17 @@ windows_compute_revents (HANDLE h, int *p_sought)
       return happened;
 
     case FILE_TYPE_CHAR:
-      ret = WaitForSingleObject (h, 0);
-      if (!IsConsoleHandle (h))
-        return ret == WAIT_OBJECT_0 ? *p_sought & ~(POLLPRI | POLLRDBAND) : 0;
-
+      // Fall through to default case for non-console, e.g. /dev/null.
+      if (IsConsoleHandle (h)) {
       nbuffer = avail = 0;
       bRet = GetNumberOfConsoleInputEvents (h, &nbuffer);
       if (bRet)
         {
           /* Input buffer.  */
           *p_sought &= POLLIN | POLLRDNORM;
           if (nbuffer == 0)
-            return POLLHUP;
+            // Having no unread events isn't an error condition.
+            return 0 /* was POLLHUP */;
           if (!*p_sought)
             return 0;
 
@@ -240,7 +239,9 @@ windows_compute_revents (HANDLE h, int *p_sought)
             return POLLHUP;
 
           for (i = 0; i < avail; i++)
-            if (irbuffer[i].EventType == KEY_EVENT)
+            // Ignore key release.
+            if (irbuffer[i].EventType == KEY_EVENT &&
+                irbuffer[i].Event.KeyEvent.bKeyDown)
               return *p_sought;
           return 0;
         }
@@ -250,13 +251,16 @@ windows_compute_revents (HANDLE h, int *p_sought)
           *p_sought &= POLLOUT | POLLWRNORM | POLLWRBAND;
           return *p_sought;
         }
+      }
+      /* fall through */
 
     default:
       ret = WaitForSingleObject (h, 0);
       if (ret == WAIT_OBJECT_0)
         return *p_sought & ~(POLLPRI | POLLRDBAND);
 
-      return *p_sought & (POLLOUT | POLLWRNORM | POLLWRBAND);
+      // Add (POLLIN | POLLRDNORM).  Why only support write?
+      return *p_sought & (POLLIN | POLLRDNORM | POLLOUT | POLLWRNORM | POLLWRBAND);
     }
 }