config auto reload has a race

Currently, we read the config files, and then start watching them. If the files are modified after we read them, but before we start watching, we will miss that. This could happen if someone edits the config just as arti is starting up (eg, their editor is being slow at saving).

To fix this, we must do things in this order: 1. identify which files we are going to read 2. start watching them 3. read them.

A complication is that the config file watching is controlled by a config item. So we don't know whether to do (2) until we have done (3). I propose to resolve this by always doing (2) and then throwing the file watchers etc. away if we discover we don't want them.

Found while working on #474 (closed)/#271 (closed)