Don't cache maintenance pages

maintenance.patch

diff --git a/includes/common.inc b/includes/common.inc
index ceac115..f3ada95 100644
--- a/includes/common.inc
+++ b/includes/common.inc
@@ -2711,6 +2711,7 @@ function drupal_deliver_html_page($page_callback_result) {
         // Print a 503 page.
         drupal_maintenance_theme();
         drupal_add_http_header('Status', '503 Service unavailable');
+        drupal_add_http_header('Cache-Control', 'no-cache, must-revalidate, post-check=0, pre-check=0');
         drupal_set_title(t('Site under maintenance'));
         print theme('maintenance_page', array('content' => filter_xss_admin(variable_get('maintenance_mode_message',
           t('@site is currently under maintenance. We should be back shortly. Thank you for your patience.', array('@site' => variable_get('site_name', 'Drupal')))))));