Add machinery to automatically setup mrouted on the control network.
This is specifically intended for elabinelab right now where configurations with an "inner control network" need a multicast router for inner frisbee to work. There is (an undocumented) NEEDMROUTED defs variable that controls whether the machinery is triggered or not. It is not documented because, though in theory this would work for a real boss setup, it has not been tested and each install would probably need a custom mrouted.conf file. For an inner elab, this variable is automatically set in defs-elabinelab if there is an inner control network. The result is that the mrouted port will be installed, it will be enabled in rc.conf, and a stub mrouted.conf file is created (to force mrouted to ignore the real control network).