This makes Site::private_address_space work like a subset of Site::local_nets,
to match many user's intuition of how we should treat site locality out of the
box. As config options, changes/redefs to Site::private_address_space propagate
to Site::local_nets, while changes to the latter don't affect the former.
A new global bit `Site::private_address_space_is_local` controls the behavior.
It defaults to true, and redefing to false brings back the original behavior.