o Added a new NSE Comm library for common network discovery tasks such

as banner-grabbing (get_banner()) and making a quick exchange of data
  (exchange()).  16 scripts were updated to use this library. [Kris]

I have *not* been able to test all of these scripts; however, I have
reviewed them and they should all work properly.  I would really like
some more testing, though :)

This commit includes scripting.xml documentation.

scripts/dns-test-open-recursion.nse

@@ -9,6 +9,7 @@ license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
 categories = {"default", "intrusive"}
 
 require "bit"
+require "comm"
 require "shortport"
 
 portrule = shortport.portnumber(53, "udp")
@@ -18,12 +19,11 @@ action = function(host, port)
     -- generate dns query, Transaction-ID 0xdead, www.wikipedia.org (type A, class IN)
 	local request = string.char(0xde, 0xad, 0x01, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x03) ..  "www" .. string.char(0x09) .. "wikipedia" .. string.char(0x03) ..  "org" .. string.char(0x00, 0x00, 0x01, 0x00, 0x01)
 
-	local socket = nmap.new_socket()
-	socket:connect(host.ip, port.number, "udp")
-	socket:send(request)
+	local status, result = comm.exchange(host, port, request, {proto="udp"})
 
-	local status, result = socket:receive();
-	socket:close()
+	if not status then
+		return
+	end
 
     -- parse response for dns flags
     if (bit.band(string.byte(result,3), 0x80) == 0x80