This function sends a DBusMessage on a DBusConnection
and sets the reply callback of the resulting DBusPendingCall,
as well as properly cancelling the pending call if anything fails.
It is inherently racy, and we have a better way to ensure that
we won't autostart the service:
dbus_message_set_auto_start()
So use that.
This commit also adds a missing call to `dbus_pending_call_unref()`
and indirectly fixes a type mismatch (`dbus_bool_t` vs. `bool`)
that was present in `is_dbus_service_running()`.