sd-radv: set only basic information on stop

sd-radv: set only basic information on stop

There are many possible reasons to stop the service;
tentative reboot of the service or the system, the router may be
revoked, and so on.
And, each situations, the availability of the previously announced
options e.g. prefixes, DNSSL and so on is not clear.
So, let's announce only the clear information, that is, the router
lifetime is zero. which indicates that the router will be invalidated
(regardless tentative or not).