Use API function to get private key

There's currently this code in update_status.module (in Drupal HEAD as well):

  $drupal_private_key = variable_get('drupal_private_key', '');
  $site_key = md5($base_url . $drupal_private_key);

The problem is, if the drupal_private_key variable hasn't been set yet, the site key becomes just md5('http://example.com'); which is not very secure. Drupal provides an API function drupal_get_private_key() which will set this variable if it isn't already.

The good news is, the chances of this variable NOT being set are very, very small, since each form calls this function when it goes to generate the token. But nonetheless, we should be calling the proper API function so that privacy is maintained.