When service_stop() handles a service in the SERVICE_AUTO_RESTART state,
it calls service_set_state() to transition it to the SERVICE_DEAD state.
However if the service failed, it should transition it to SERVICE_FAILED
instead, which will trigger its OnFailure units. To achieve this, we now
call service_enter_dead() in place of service_set_state(), which will
transition the service to either SERVICE_DEAD or SERVICE_FAILED as is
appropriate.

Also, some misleading comments are adjusted: service_stop() is not only
called on a user request, but also during an automatic restart in order
to handle dependencies.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=45511