When there is a transition in permitted nodes, apply it to trackexithosts map

When there is a transition in permitted nodes, apply it to trackexithosts map

IOW, if we were using TrackExitHosts, and we added an excluded node or
removed a node from exitnodes, we wouldn't actually remove the mapping
that points us at the new node.

Also, note with an XXX022 comment a place that I think we are looking
at the wrong string.