%R is already used in service manager specifier expansion (cgroup root),
hence use a different char, that was so far not used.
Follow-up for: 6ceb0a4094
This specifes two new optional fields for /etc/os-release:
IMAGE_VERSION= and IMAGE_ID= that are supposed to identify the image of
the current booted system by name and version.
This is inspired by the versioning stuff in
https://github.com/systemd/mkosi/pull/683.
In environments where pre-built images are installed and updated as a
whole the existing os-release version/distro identifier are not
sufficient to describe the system's version, as they describe only the
distro an image is built from, but not the image itself, even if that
image is deployed many times on many systems, and even if that image
contains more resources than just the RPMs/DEBs.
In particular, "mkosi" is a tool for building disk images based on
distro RPMs with additional resources dropped in. The combination of all
of these together with their versions should also carry an identifier
and version, and that's what IMAGE_VERSION= and IMAGE_ID= is supposed to
be.
As described in #15603, it is a fairly common setup to use a fqdn as the
configured hostname. But it is often convenient to use just the actual
hostname, i.e. until the first dot. This adds support in tmpfiles, sysusers,
and unit files for %l which expands to that.
Fixes#15603.
We probably can migrate even more, but for now let's just migrate those
which have the 1:1 identical text everywhere.
(Also, let's add the % entry to all specifier tables)
