Do a connect on rpc-grind UDP sockets to invoke socket_lock.

This connect should be side effect–free except for calling socket_lock,
which prevents the creation of an excessive number of sockets. Not using
the lock was causing "Too many open files" errors.

We should have a cleaner general solution for this, and not require
scripts to "connect" their unconnected UDP sockets. I seem to remember
that there was a good reason for not enforcing the lock on socket
creation, but only on connect, as we do.

http://seclists.org/nmap-dev/2012/q4/435

nselib/rpc.lua

@@ -187,11 +187,15 @@ Comm = {
             local resvport = math.random(1, 1024)
             socket = nmap.new_socket("udp")
             status, err = socket:bind(nil, resvport)
-            if status then break end
-            socket:close()
+            if status then
+              status, err = socket:connect(host, port)
+              if status or err == "TIMEOUT" then break end
+              socket:close()
+            end
           end
         else
           socket = nmap.new_socket("udp")
+          status, err = socket:connect(host, port)
         end
       end
       if (not(status)) then