Tor already has `DisableNetwork`, an option that prevents network connections. But when testing legacy/trac#15775 (Fallback Directories), it would be useful to be able to disable connections to the authorities only. (Like the attached torrc.block_authorities file.) This would help us ensure bootstrap works without authorities. `DisableDirectDirAuthConnection`? When testing legacy/trac#8374, it could be useful to disable IPv4, so that we can ensure bootstrap works with only IPv6. This would be like `ClientUseIPv6` and `ClientPreferIPv6ORPort`, but applying strictly to the entire Tor process: `DisableIPv6Network`. And it might also be useful to disable IPv6, if IPv6 becomes active by default: `DisableIPv4Network`. This would help us cover the IPv4/IPv6 testing combinations listed at the bottom of: https://lists.torproject.org/pipermail/tor-relays/2015-June/007109.html