Prop 312: 3.2.6. Add an AddressDisableIPv6 torrc option

Should be done after the first IPv6 address guessing feature: legacy/trac#33238 (moved) or legacy/trac#33240 (moved).

Relays (and bridges) that have a reachable IPv6 address, but that address is unsuitable for the relay, need to be able to disable IPv6 address resolution.

Currently, relays are required to have an IPv4 address. So if the guessed IPv4 address is unsuitable, operators can set the Address option to a suitable IPv4 address. But IPv6 addresses are optional, so relay operators may need to disable IPv6 entirely.

We propose a new torrc-only option, AddressDisableIPv6. This option is set to 0 by default. If the option is set to 1, tor disables IPv6 address resolution, IPv6 ORPorts, IPv6 reachability checks, and publishing an IPv6 ORPort in its descriptor.

See proposal 312, section 3.2.6: https://gitweb.torproject.org/torspec.git/tree/proposals/312-relay-auto-ipv6-addr.txt#n498

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