[NSE] Fixed a bug I just found where NSE would report that a --script

expression did not match files when it did. This happens when an expression
matches a set of scripts that were loaded by other expressions first. A simple
example is the same expression with different capitalization (considered
to be two different expressions):

batrick@waterdeep:~/nmap/svn/nmap$ ./nmap -d2 -v --script default,DEFAULT localhost > /dev/null
NSE: failed to initialize the script engine:
./nse_main.lua:365: 'DEFAULT' did not match a category, filename, or directory
stack traceback:
    [C]: in function 'error'
    ./nse_main.lua:365: in function 'get_chosen_scripts'
    ./nse_main.lua:498: in main chunk
    [C]: ?

QUITTING!

nse_main.lua

@@ -344,9 +344,9 @@ local function get_chosen_scripts (rules)
         assert(t == "file", filename.." is not a file!");
         if not files_loaded[path] then
           chosen_scripts[#chosen_scripts+1] = Script.new(path);
-          used_rules[rule_table.original_rule] = true;
           files_loaded[path] = true;
         end
+        used_rules[rule_table.original_rule] = true;
       end
     end
   end