Allow hook_exit to read the status code being sent out from drupal_goto

In drupal 7 this is not an issue due to the $http_response_code being sent out via drupal_alter in drupal_goto. Drupal 6's drupal_goto doesn't have this option and from my experiments with php header functions, it is impossible to get the $http_response_code from the header.

So I'm proposing that we pass http_response_code along. So this

module_invoke_all('exit', $url);

would become this

module_invoke_all('exit', $url, $http_response_code);