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/housekeeper.h

@@ -51,11 +51,34 @@ typedef struct hktask
     struct hktask *next;      /*< Next task in the list */
 } HKTASK;
 
-extern void hkinit();
+/**
+ * Initialises the housekeeper mechanism.
+ *
+ * A call to any of the other housekeeper functions can be made only if
+ * this function returns successfully.
+ *
+ * @return True if the housekeeper mechanism was initialized, false otherwise.
+ */
+extern bool hkinit();
+
+/**
+ * Shuts down the housekeeper mechanism.
+ *
+ * Should be called @b only if @c hkinit() returned successfully.
+ *
+ * @see hkinit hkfinish
+ */
+extern void hkshutdown();
+
+/**
+ * Waits for the housekeeper thread to finish. Should be called only after
+ * hkshutdown() has been called.
+ */
+extern void hkfinish();
+
 extern int  hktask_add(const char *name, void (*task)(void *), void *data, int frequency);
 extern int  hktask_oneshot(const char *name, void (*task)(void *), void *data, int when);
 extern int  hktask_remove(const char *name);
-extern void hkshutdown();
 extern void hkshow_tasks(DCB *pdcb);
 
 MXS_END_DECLS

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>