Use O_DIRECT, but only if available.

TODO: The kernel version should be looked up at startup and made
      generally available so that it is readily available for
      anybody interested.

server/core/maxscale/messagequeue.hh

@@ -116,6 +116,21 @@ public:
     typedef MessageQueueHandler Handler;
     typedef MessageQueueMessage Message;
 
+    /**
+     * Initializes the message queue mechanism. To be called once at
+     * process startup.
+     *
+     * @return True if the initialization succeeded, false otherwise.
+     */
+    static bool init();
+
+
+    /**
+     * Finalizes the message queue mechanism. To be called once at
+     * process shutdown, if the initialization succeeded.
+     */
+    static void finish();
+
     /**
      * Creates a @c MessageQueue with the provided handler.
      *