Clean up string concatenations

Building a string with var = var .. "something" has miserable time
complexities. This commit cleans up a lot of that in scripts, focusing
on packing of data with bin.pack and concatenations within loops.
Additionally, a few instances were replaced with string.rep

scripts/http-fileupload-exploiter.nse

@@ -177,11 +177,12 @@ end
 local prepareRequest = function(fields, fieldvalues)
 
   local filefield = 0
-  local req = ""
+  local req = {}
   local value
 
   for _, field in ipairs(fields) do
     if field["type"] == "file" then
+      -- FIXME: What if there is more than one <input type="file">?
       filefield = field
     elseif field["type"] == "text" or field["type"] == "textarea" or field["type"] == "radio" or field["type"] == "checkbox" then
       if fieldvalues[field["name"]] ~= nil then
@@ -189,11 +190,11 @@ local prepareRequest = function(fields, fieldvalues)
       else
         value = "SampleData0"
       end
-      req = req .. '--AaB03x\nContent-Disposition: form-data; name="' .. field["name"] .. '";\n\n' .. value .. '\n'
+      req[#req+1] = ('--AaB03x\nContent-Disposition: form-data; name="%s";\n\n%s\n'):format(field["name"], value)
     end
   end
 
-  return req, filefield
+  return table.concat(req), filefield
 
 end