Should system-arti use a different SOCKS port from user-arti?

Currently arti generally runs under the current user, and reads config files from $HOME. But it seems that system packagers are interested in packaging arti to run as a system service where all users share the same arti proxy and circuits, like how c-tor works.

I think that it's still useful to be able to run arti locally as the current user, but if a user-arti and system-arti use the same default configuration, then they would both try to use the same SOCKS port. It would be not so nice if people apt install arti, then try to run arti and it fails because a system service started in the background with the same SOCKS port. I think now would be the time to change this before the system packages become widely used, if we do want to change it.

I think we should recommend that packagers use a non-default SOCKS port for the arti daemon, and we should also make this change for our own deb package in debian/. Maybe the arti daemon should use tor's SOCKS port of 9050?

Related: #1675