Enabling user daemons and D-Bus activation

@jamesh I ran into a problem where my user daemon doesn’t start upon first install and doesn’t stop after removal, but only when running on Manjaro with xorg. I think refreshes are a problem too. I see some discussion in https://github.com/snapcore/snapd/pull/5822 about how snapd can’t stop user services and some systemd versions don’t work well with the user-daemons experimental feature. I was wondering if you could confirm this is the issue I am facing.

I tested my (still unreleased) snap on other linux variants (see First snap - testing across different linux variants) and the user daemon worked really well on all except manjaro with org (e.g. xfce). It seems that there is something in this particular setup that’s different from the others with respect to how snapd works with systemd.