[packagekit] UID and CommandLine in a backend

[Richard Hughes]

For the Zif backend I needed to know the user ID and the command line
of each transaction. To do this, I've added two new properties for
both the uid of the user who fired the PK method, and a fake command
line to PkBackend.

This might be useful to other backends, but if this doesn't interest
you, then please ignore this email. :)

Thanks,

Richard.