Make routerset_contains_router() support IPv6
In router_should_check_reachability(), we test to see if the current relay is in ExcludeNodes.
But ExcludeNodes doesn't support IPv6, so if a user excludes our IPv6 address, we'll just ignore that setting.
This is not a required task.
But we should probably be explicit about ignoring IPv6 in the man page.
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