dbus: make use of [Install] unit sections
    
    Rather than installing symlinks during the installation manually, put
    in [Install] sections into the units. This has the same effect, but
    allows changing the installation conditions via systemd-presets, which
    are provided by the 'fedora-release' package.
    
    Signed-off-by: David Herrmann <dh.herrmann@gmail.com>