default clients, including tor browser, fail to bootstrap on an ipv6-only network

If you run your Tor Browser on a network that only has ipv6, it will never bootstrap.

There are ipv6 addresses in the fallback-dirs list, but it seems that Tor never tries them.

Bug reported at CCC by an ISP operator from Ukraine, and confirmed today by @trinity-1686a.

trinity says that if you set ClientPreferIPv6ORPort 1, then things do work. But I have vague memories of us setting prefer-ipv6 by default in Tor Browser and regretting it and turning it off again -- perhaps because it never tries ipv4 in some circumstances that happen in practice.