lightdm: Specify D-Bus bus name in service unit

Do it [as Debian does it]
(https://github.com/canonical/lightdm/blob/ff1c38c0616b3df66a5002ed7f531d53f218dfab/debian/lightdm.service).

From manual page systemd.service(5):

> BusName=
>     Takes a D-Bus bus name that this service is reachable as. This option is
>     mandatory for services where Type= is set to dbus.

lightdm.be0

@@ -88,6 +88,7 @@ EOF
 	[Service]
 	ExecStart=/usr/bin/lightdm
 	Restart=always
+	BusName=org.freedesktop.DisplayManager
 	UMask=007
 
 	[Install]