Implement fhandler_dev_null::write to workaround a problem with NUL

Windows NUL device returns only the lower 32 bit of the number of
bytes written.  Implement a fake write function to ignore the underlying
NUL device.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fhandler.h

@@ -1682,6 +1682,8 @@ class fhandler_dev_null: public fhandler_base
     copyto (fh);
     return fh;
   }
+
+  ssize_t __stdcall write (const void *ptr, size_t len);
 };
 
 class fhandler_dev_zero: public fhandler_base