Reauthorize against a polkit agent

Here's the reauthorize work completed. No support for sudo yet, that'll come later by routing those requests through the polkit agent.

There's documentation in ```doc/HACKING.md``` on how to enable sshd for reauthorization when using remote hosts.

You can see that the implementation changes half way through the branch. I'd like to leave this history there as documentation of the various choices that were made. All commits build, even those that are later replaced, and pass make check.

This depends on pull requests #530, #529, #509, and needs to be rebased on those once they are merged.