systemctl: make sure "reboot", "suspend" and friends are always asynchronous

systemctl: make sure "reboot", "suspend" and friends are always asynchronous

Currently, "systemctl reboot" behaves differently in setups with and
without logind. If logind is used (which is probably the more common
case) the operation is asynchronous, and otherwise synchronous (though
subject to --no-block in this case). Let's clean this up, and always
expose the same behaviour, regardless if logind is used or not: let's
always make it asynchronous.

It might make sense to add a "--block" mode in a future PR that makes
these operations synchronous, but this requires non-trivial work in
logind, and is outside of the scope of this change.

See: #6479