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/core/unit-printf.c

@@ -291,6 +291,7 @@ int unit_full_printf(const Unit *u, const char *format, char **ret) {
 
                 { 'm', specifier_machine_id,               NULL },
                 { 'H', specifier_host_name,                NULL },
+                { 'l', specifier_short_host_name,          NULL },
                 { 'b', specifier_boot_id,                  NULL },
                 { 'v', specifier_kernel_release,           NULL },
                 {}