Reload configuration on SIGHUP, or something

The arti executable should, on Unix, honour SIGHUP, and respond by reloading the configuration.

IHNI what would the idiomatic way on Windows.

There should perhaps be other ways to achieve this too. I don't think we have anything like a control socket (and perhaps we don't want to add one). But I think that's a future thing that doesn't want to be on this milestone.

(as per !280 (comment 2774215))