WiP: Add uniter changes to support reboot

This PR adds the uniter changes needed to support reboot.

Depends on:

https://github.com/juju/juju/pull/707
https://github.com/juju/juju/pull/728
https://github.com/juju/juju/pull/744
https://github.com/juju/juju/pull/782
https://github.com/juju/juju/pull/833

doesn't this just mean we have to improve how juju-run works to the point where it does record the pid?