homed: stop before stopping dbus

Otherwise, systemd-homed-active.service will fail to deactivate all
homes because homectl can no longer talk to homed if dbus stops first.
As a result, /home cannot be umounted.

Doing this on systemd-homed-active.service instead works as well, but
systemd-homed will exit 1 if dbus is already shut down.

(cherry picked from commit e00a25a7)
(cherry picked from commit 11f3040d)
(cherry picked from commit e2012e0d)