Delivers a "site is under maintenance" message to the browser.
Page callback functions wanting to report a "site offline" message should return MENU_SITE_OFFLINE instead of calling drupal_site_offline(). However, functions that are invoked in contexts where that return value might not bubble up to menu_execute_active_handler() should call drupal_site_offline().
function drupal_site_offline() {
drupal_deliver_page(MENU_SITE_OFFLINE);
}