Merge lp://staging/~jamesh/thumbnailer/dbus-aa-credentials into lp://staging/thumbnailer/devel

Track the credentials (user ID, AppArmor label) of clients connecting to the D-Bus service. This is not yet used to make security decisions.

This is the first step of the changing our security policy to rely on aa_query_label().

I've introduced a new step for the request handler to determine the AppArmor security context of the client. At the moment we're only printing it out in a log message, but eventually this information will be pushed down to the ThumbnailRequest where it can be used to make the security decision.

You can test this on the desktop by running thumbnailer-service in one terminal and in another run something like:

    aa-exec -p $profile thumbnailer-admin get $filename outdir/

You can get a list of available profiles on the system with "sudo aa-status". When run on the phone, you should see the labels for confined clients.

To avoid excessive GetConnectionCredentials() calls to the bus daemon, we cache the results in the CredentialsCache class.