units: starting suspend.target should not fail when suspend is successful (#6678)

and the same for hibernate.target and hybrid-sleep.target.

Tested with both sucessful and unsuccessful suspends.  The result of the
start job was correct in both cases.  Closes #6419 (a regression in v233
and v234).

> suspend is unsual for a target, because it has to stop itself once it's
> started. Otherwise you couldn't start it again, so you could only suspend
> once! Currently that's implemented using BindsTo=systemd-sleep.service.
> Meaning it pulls in systemd-sleep.service to do the actual suspend, and
> then de-activates afterwards. But the behaviour of BindsTo was changed
> recently (not without some issues during development) - maybe this bug
> is caused by poettering/systemd@631b676 which I think was added in
> release v233.
>
> sleep.target (see man systemd.special) has the same need, but it
> implements it differently. It simply has StopWhenUnneeded=yes.

This commit switches suspend.target etc. to the approach used by
sleep.target.