core/service: actually allow to "hurry up" auto restarts

core/service: actually allow to "hurry up" auto restarts

unit_start() advertises that start requests don't get suppressed,
so that it could be used to manually speed up auto restarts.
However, service_start() so far rejected this, stating that
clients should issue restart request in order to trigger
BindsTo=/OnFailure=.

That seems to be a red herring though, because for a long time
the service states between auto-restarts were buggy (#27594).
With the introduction of RestartMode=direct, the behavior
is sane again and customizable, hence I see no reason to refuse
this anymore. Whether those deps are triggered solely depends
on RestartMode= now.

Plus, filter out some intermediate states that should never
be seen in service_start().

Fixes #33890