This makes things a bit simpler and the build a bit faster, because we don't
have to rewrite files to do the trivial substitution. @rootbindir@ is always in
our internal $PATH that we use for non-absolute paths, so there should be no
functional change.
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)