An explicit --user switch is necessary because for the user@0.service instance
systemd-tmpfiles is running as root, and we need to distinguish that from
systemd-tmpfiles running in systemd-tmpfiles*.service.
Fixes#2208.
v2:
- restore "systemd-" prefix
- add systemd-tmpfiles-clean.{service,timer}, systemd-setup.service to
systemd-tmpfiles(8)
I want to add presets/user/ later. This mirrors the layout for units:
we have units/ and units/user. The advantage is that we avoid having yet
another directory at the top level.