systemctl: make sure legacy "reboot", "suspend" and friends are always asynchronous (#8848)

Currently, "reboot" behaves differently in setups with and without logind.
If logind is used (which is probably the more common case) the operation
is asynchronous, we should behave in the same way as "systemctl <verb>".
Let's clean this up, and always expose the same behaviour, regardless if
logind is used or not: let's always make it asynchronous.

See: #6479
Fixes: commit 130246d2