D-Bus session bus address not serialised

Bug 1235649 introduced the ability to have the Session Init connect to a specified D-Bus bus. This works fine except that when the Session Init is re-exec'd the address passed via 'initctl notify-dbus-address' is lost, since it is not serialised. Hence, post-re-exec, the Session Init does not attempt to reconnect automatically as it should if the address was known prior to the re-exec.