let ctools_include support no subdirectories (NULL)

Currently you cant use ctools_include if the file is not in a subdir, which is kind of unhandy.

fairly easy extension:

function ctools_include($file, $module = 'ctools', $dir = 'includes') {
  static $used = array();
  $ref = $used[$module];
  
  if ($dir != NULL) {
    $ref = $used[$module][$dir];
  }  
    
  if (!isset($ref[$file])) {
    if ($dir != NULL) {    
      require_once './' . drupal_get_path('module', $module) . "/$dir/$file.inc";
      $used[$module][$dir][$file] = true;
    }
    else {
      require_once './' . drupal_get_path('module', $module) . "/$file.inc";
      $used[$module][$file] = true;
    }
  }
}