Default config location should include a Debian-style arti.d

Long experience in Debian shows that this arrangement, where a program reads any suitably-named files in a config directory named after the program (as well as reading a trad single file) is very convenient for situations where possible multiple different parts of the system might want to provide bits of the config.

Unless there is a good reason not to we should do this in arti too. CC @nickm for his opinion before I implement this...