Replace print() calls with stdnse.print_debug()

NSE scripts should not send data to stdout with print(). One exception
was not altered: url-snarf states in its documentation that urls are
sent to stdout by default, with a script-arg option to save to a file
instead.

scripts/dns-brute.nse

@@ -248,7 +248,7 @@ action = function(host)
 				end
 				file:close()
       else
-				print("dns-brute: Hostlist file not found. Will use default list.")
+				stdnse.print_debug("dns-brute: Hostlist file not found. Will use default list.")
 			end
     end
 		if (not hostlist) then hostlist = HOST_LIST end