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)