Add %l as specifier for the hostname without any domain component

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.

src/basic/hostname-util.h

@@ -9,6 +9,7 @@
 bool hostname_is_set(void);
 
 char* gethostname_malloc(void);
+char* gethostname_short_malloc(void);
 int gethostname_strict(char **ret);
 
 bool valid_ldh_char(char c) _const_;