Change clients to automatically use IPv6 if they can bootstrap over it
Tor currently defaults to avoiding IPv6 directories and ORPorts. When legacy/trac#8374 (moved) and legacy/trac#4483 (moved) are implemented, we should change this default, especially for those clients that can bootstrap successfully over IPv6.
I suggest we add an "auto" option, and make it the default. It would mean that the client uses whichever IP version it bootstrapped over.
ClientUseIPv6 0 ClientPreferIPv6ORPort 0