testing/shutdown-clear-machine-id: new aport

Create service to remove machine-id on shutdown, to get rid of a persistent UID that isn't strictly necessary. Of course this service is opt-in with the package. I know there's a lot of FUD about machine-id but I think this is just a nice elimination of another point of entropy.

Devuan regenerates the machine-id on every boot in it dbus package

ChromiumOS apparently rewrites machine-id pretty frequently too. source

I've not added this as a subpackage to dbus because maybe that's too much downstream patching, or the maintainers wouldn't want this, but if that's a better place for it I can add it there instead.