units: improve Description= for systemd's own busname unit

bus: provide org.freedesktop.systemd1.busname for systemd --user

busname: introduce Activating directive Add a new config 'Activating' directive which denotes whether a busname is actually registered on the bus. It defaults to 'yes'. If set to 'no', the .busname unit only uploads policy, which will remain active as long as the unit is running.