service: don't enter a second SIGTERM/SIGKILL cycle if no ExecStopPost= process is defined

It won't help if the main process is still there and there is no new
process to kill.
update TODO
mount: also exclude /usr from unmount at shutdown
systemctl: also highlight a load state of "not-found" as red

"not-found" is a recently added load state and was previously just a
special case of "error". Since it also indicates a load error we should
also highlight it red, the same way as "error" was treated before.