Check/enforce path restrictions for each path position

For the vanguard torrc options, we may want to check that each layer has at least one node from a different /16 and different node family than others in that layer, to ensure that a path can always be built using the vanguard set.

We may also want to do the same thing for Tor's Primary Guard set from Prop271, to ensure that an adversary can't force the user to pick guards randomly from Sampled Guards.

Doing both of these things at once should allow us to drop legacy/trac#24487 (moved).

See also: https://gitweb.torproject.org/torspec.git/tree/proposals/291-two-guard-nodes.txt#n33