Headless server: Add support for setting a port from commandline

Manual editing of a config file for setting a port is not ideal.
Solution: add a set-port command to save a port and also a
--port option to allow setting a temporary port during startup

YACReaderLibrary/server/startup.h

@@ -26,7 +26,7 @@ public:
     /** Constructor */
     Startup();
     /** Start the server */
-    void start();
+    void start(quint16 port = 0);
     /** Stop the server */
     void stop();