secure default installation

By setting port 0 by default we get a client only installation by
default.
The server behavior can be changed by setting the port parameter
to 123
* http://chrony.tuxfamily.org/manual.html#port-directive
* http://listengine.tuxfamily.org/chrony.tuxfamily.org/chrony-users/2014/01/msg00008.html