hostnamed: if polkit authentication fails for Varlink Describe() call, don't reply...

hostnamed: if polkit authentication fails for Varlink Describe() call, don't reply to client with an error

The logic of the Describe() call was supposed to be: if we can acquire
the PK priv to get the product UUID then let's return the product UUID,
and if we cannot then return the data without it.

This didn't work however, since the polkit varlink glue would
immediately propagate the error it acquired from polkit its own client.
Let's turn this off, optionally, so that hostnamed can handle this
nicely.