bus: don't switch to kdbus if not requested

bus: don't switch to kdbus if not requested

Whenever systemd is re-executed, it tries to create a system bus via
kdbus. If the system did not have kdbus loaded during bootup, but the
module is loaded later on manually, this will cause two system buses
running (kdbus and dbus-daemon in parallel).

This patch makes sure we never try to create kdbus buses if it wasn't
explicitly requested on the command-line.