add some FD_SETSIZE research from David

docs/TODO

@@ -196,6 +196,46 @@ o [Web] Consider adding training/introduction videos to the Nmap site
 
 o Change Nsock to give an error if you try to FD_SET a fd larger than
   FD_SETSIZE. [Brandon]
+  o Some research from David:
+     We have help off on this change because of Windows portability
+     problems. The Windows fd_set works differently than the Unix
+     fd_set. In Unix, FD_SETSIZE (which is typically 1024) is both the
+     maximum number of file descriptors that can be in the set and one
+     greater than the greatest file descriptor number that can be
+     set. In other words, we want to bail out whenever someone tries
+     to FD_SET file descriptor 1060, for example. But on Windows it's
+     different: FD_SETSIZE is only 64, but any file descriptor
+     numbers, no matter how great, may be stored in the set. Windows
+     socket descriptors are typically greater than 1023, but you can
+     only have 64 of them in the set at once.
+
+     So the fix on Unix would be
+     --- nsock/src/nsock_core.c	(revision 15214)
+     +++ nsock/src/nsock_core.c	(working copy)
+     @@ -97,6 +97,7 @@
+      do { \
+        assert((count) >= 0); \
+        (count)++; \
+     +  assert((sd) < FD_SETSIZE); \
+        FD_SET((sd), (fdset)); \
+        (max_sd) = MAX((max_sd), (sd)); \
+        return 1; \
+     @@ -107,6 +108,7 @@
+        assert((count) > 0); \
+        (count)--; \
+        if ((count) == 0) { \
+     +    assert((sd) < FD_SETSIZE); \
+          FD_CLR((sd), (fdset)); \
+          assert((iod)->events_pending > 0); \
+          if ((iod)->events_pending == 1 && (max_sd) == (sd)) \
+
+     But that doesn't work on Windows (I just tried it) because even
+     the smallest socket descriptor is bigger than FD_SETSIZE, 64.
+     Really we're trying to accomplish two different things on the two
+     platforms: On Unix we must not store a file descriptor greater
+     than 1023, no matter how many or how few other descriptors have
+     been set. On Windows we must not set more than 64 descriptors at
+     a time, no matter what their descriptor number happens to be.
 
 o Change Nsock so that it is able to take advantage of more modern
   interfaces to dealing with large sockets, rather than just select.