MEDIUM: stats: add the ability to kill sessions from the admin interface

It was not possible to kill remaining sessions from the admin interface,
which is annoying especially when switching to maintenance mode. Now it's
possible.
diff --git a/include/types/proto_http.h b/include/types/proto_http.h
index 3457a2a..76c8e6b 100644
--- a/include/types/proto_http.h
+++ b/include/types/proto_http.h
@@ -264,6 +264,7 @@
 	ST_ADM_ACTION_ENABLE,
 	ST_ADM_ACTION_STOP,
 	ST_ADM_ACTION_START,
+	ST_ADM_ACTION_SHUTDOWN,
 };
 
 /* status codes available for the stats admin page */