Agent Bakery and update agent as non-root

Today it's possible to have the agent bakery create an agent that is not running as root. However this does not allow the agent to update itselves.

It would be useful if the new implemented agent-controller could handle the agent update process (and not cmk-update-agent) by creating a separate process.

There are also annoying permissions issues where a lot of files and directories are owned by root when they should be owned by the user who runs the agent.