portablectl: block when stopping a unit on detach (--now)

portablectl: block when stopping a unit on detach (--now)

If portablectl detach --now is used, there's a possible race condition
where the unit is not stopped in time before the detach is attempted,
which causes it to fail.
Add a DBUS call to block after starting/stopping if --now is passed,
and add a --no-block parameter to skip it optionally when starting,
since it is not necessary in that case for correct functioning.