Hardcoding a DISPLAY variable is a bad idea because it means that dunst
will only run on X11, and only for that specific configuration.
FWIW, this actually applies to any desktop app run via systemd:
1. `DISPLAY` should be set by `systemctl --user set-environment` elsewhere.
2. `DISPLAY=:0` is not universally valid.
3. This breaks dunst if attempting to run wayland, and requires manually
starting it (eg: not via systemd), or editing the file.
multi-user.target isn't a valid user target and it doesn't make sense to run
dunst as a system service.
Eventually, this unit should probably be converted to use
graphical-session.target but it's a bit unclear how that's supposed to work
right now.
