Add torrc option to disable default fallback directories

There's no easy way to disable the default fallback directories.

Tails depends on using the authorities for its consensuses as part of its time syncing proof: https://tails.boum.org/contribute/design/Time_syncing/#index1h2

We should provide an option to disable the default fallback directories entirely. (It should also conflict with any FallbackDir lines.)

Default fallbacks are also disabled when any other fallbacks are added, or when using non-default authorities.