Add configuration option for indicating if a directory authority is on IPv6 or not
Background from legacy/trac#5534 (moved):
TODO: Add a configuration option telling tor if it's supposed to have IPv6 connectivity. This will be something like AuthorityHasIPv6Connectivity 0|1|auto. Clients and ordinary relays will have to detect this.
This should be an option for authorities only for use until we have good automatic detection of IPv6 connectivity.
The idea is that an authority with this option set to 0 will emit "a" lines with IPv6 relay addresses without performing reachability tests. The reasoning behind it is that relays on IPv6 shouldn't be punished by directory authorities not on IPv6.
Regarding its default value, I think that 'auto' would be a good default. Once we have automatic detection in place, leaving this option out will make no difference. In the meantime 'auto' will turn into '0' and authorities will by default let "a" lines through.
Regarding automatic detection, I wonder how we can do that without leaking information. Is it possible? If not, how much can we leak? Should we add more config option(s) for enabling more leaky strategies?