Wait until housekeeper has finished

When calling hkshutdown(), only a shutdown flag will be set.
In main() we will then actually wait for the housekeeper thread
to finish before exiting.

include/maxscale/semaphore.h

@@ -0,0 +1,22 @@
+#pragma once
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl.
+ *
+ * Change Date: 2019-07-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+/**
+ * @file semaphore.h  Semaphores used by MaxScale.
+ */
+
+// As a minimal preparation for other environments than Linux, components
+// include <maxscale/semaphore.h>,  instead of including <semaphore.h>
+// directly.
+#include <semaphore.h>