Should the [system] configuration belong to `arti`, not `arti-client`?

Right now, the [system] configuration heading is used for the max_file_descriptors value, which we set in arti. But the section itself is part of TorClientConfig.

This violates the encapsulation we have in mind: if there is functionality that gets configured in TorClientConfig, it should be implemented in arti-client or below so that all Arti users get it.

Personally I think it makes more sense to have max_file_descriptors implemented in arti: it isn't very usual for libraries to mess with their applications' resource limits. If that's the case, it doesn't belong in TorClientConfig.

We might want to defer this change until after #285 (closed) is done, since that ticket will substantially refactor our configuration.