`Address:is_local` would return `false` for all DNS names. This is
incorrect, with one counterexample being the name "localhost", which
generally resolves to a local (usually loopback) address.
The function is changed to catch "localhost", but also the top level
domain ".localhost", which is reserved in RFC 2606, Section 2.
`Address::is_routable` would return `true` for all DNS names. While it
is much harder to decide global routeability based on a domain name, as
these usually have to be resolved to an address before being able to
judge routability, there is one particular class of names, namely local
ones (see above), which are not globally routable.
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