privileged-helper branch

[Alexander Larsson]

I just pushed this branch:

https://github.com/alexlarsson/xdg-app/tree/privileged-helper

It adds a system dbus service,
called org.freedesktop.XdgApp.SystemHelper which uses policykit to
allow you to do installs and updates into the system xdg-app instance.

The way it works is that the client creates a temporary ostree repo
with "parent" set to point to the system repo, which means we inherit
the objects and the remote configuration from the system repo. Then we
do an ostree pull into this, and when we have all the files downloaded
we ask the system helper to pull this into the system repo (after
verifying gpg signatures and sha256 checksums).

However, this is basically the first time ever I looked at policykit in
anger, so I would very much appreciate someone reviewing this code.
Anyone up for it?

-- 
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 Alexander Larsson                                            Red Hat, Inc 
       alexl at redhat.com            alexander.larsson at gmail.com 
He's a lonely one-eyed dwarf gone bad. She's a blind cigar-chomping 
mermaid from aristocratic European stock. They fight crime!