config: sort out default config file location etc.

In #285 (closed):

The knowledge of the default config file location(s) and sources should be (exposed) in arti-client (not arti), with the implication that we hope that most embedders will use it. (tor-config can continue to be the actual implementation of env vars, default path lookup, etc.)

Perhaps in fact this ought to be in arti-client rather than arti-config. We should:

• Decide whether arti-config still has a reason to exist separately from arti-client, and what the API for "use default config files" ought to be like
• Consider "Instead of requiring arti_client to use compile-time defaults, give TorClientBuilder the ability to tell the client to load from the default locations"
• Change the default config file location
• Ensure that knowledge of the default config file location is appropriately centralised
• Ensure that an application can extend the configuration (ie, read from the default locations, but also parse additional sections in those very same default files) (!529 (merged))