Set the close_handlers member in worker Thread objects. The lack of this caused

them to use the close_handlers table of the parent thread that spawned them
instead. See http://seclists.org/nmap-dev/2010/q3/804 for more details.

nse_main.lua

@@ -692,6 +692,7 @@ local function run (threads, scantype)
       type = current.type,
       parent = current.parent,
       info = format("'%s' worker (%s)", current.short_basename, tostring(co));
+      close_handlers = {},
       -- d = function(...) end, -- output no debug information
     };
     local thread_mt = {