Lua 5.2 upgrade [1] for NSE.

[1] http://seclists.org/nmap-dev/2012/q2/34

scripts/dns-recursion.nse

@@ -1,3 +1,9 @@
+local bit = require "bit"
+local comm = require "comm"
+local nmap = require "nmap"
+local shortport = require "shortport"
+local string = require "string"
+
 description = [[
 Checks if a DNS server allows queries for third-party names. It is
 expected that recursion will be enabled on your own internal
@@ -16,9 +22,6 @@ license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
 
 categories = {"default", "safe"}
 
-require "bit"
-require "comm"
-require "shortport"
 
 portrule = shortport.portnumber(53, "udp")