service: use the right timeout for stop processes we fork
update TODO
service: allow services of Type=oneshot that specify no ExecStart= commands

This is useful for services that simply want to run something on
shutdown, but not at bootup. They should only set ExecStop= but leave
ExecStart= unset.
install: simplify usage of _cleanup_ macros
systemctl: in list-unit-files, always show legend, even if we know about no unit files
update TODO