Fix sleep/suspend sometimes not working
... from ksmserver-logout-greeter, by making the DBus calls synchronous.
As is already noted for the shutdown and reboot cases, systemd-logind will check the calling process for credentials. This fails when the calling process has already exited.
In the case of requesting sleep/suspend/hibernate, this very thing happens specifically from ksmserver-logout-greeter - the (optional?) choices screen shown for a logout/sleep/power action.
See also the bug report where this all started out: https://bugs.gentoo.org/818124