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

how can ctx.pid be 0 here?