Allow routersets to include/exclude nodes by IPv6 address

Right now, routerset_contains_*() and friends only looks at a single tor_addr_t. But nodes now can have more than one address.

The simple rule here would be to have a node considered to be a member of a routerset if any of its addresses is contained in the routerset. That would make ExcludeNodes work right, but could get surprising behavior with ExitNodes and the like.

A less simple rule would be to have slightly different semantics for 'include' and 'exclude' routersets, but I don't much like that option: it also seems likely to surprise.

The least simple option would be to treat a node as partially in a set if one of its addresses is included and another isn't. That's a worst-case option IMO: it's likely to be complex and error-prone.