man: Document the actual behaviour of ExecStopPost

The documentation makes it sound like ExecStopPost is only run when
stopping the service with `systemctl stop foo.service`

However, that is not the case, as it also gets run when the service
unexpectedly exists, crashes, or gets SIGKILLed.
build-sys: use -f with ln -s

Parallel builds would sometimes try to recreate the link,
and fail since 'ln -s' would refuse to overwrite.