Automatically detect and tolerate IPv6-only environments

Arti should be able to detect when it's running without the ability to connect to an IPv4 address, and behave reasonably in those cases.

This ticket relates to #329 (closed) and #93 (closed); although "IPv6-only" is one common kind of address restriction that a client might have, it's not the only such restriction.

Also, detection can be tricky: a lack of IPv6 interface means that we can't connect to IPv6, but the presence of an IPv6 interface doesn't mean that it's usable.