Show user who authenticated reconfigure in Kea
The goal of this ticket is to show the username who authorized reconfigure (or say that auth was disabled or not possible to determine, because it was a server restart or a signal was sent).
This is a follow-up to #353 (comment 164884).
Once kea#1590 (closed) is implemented, we should retrieve this information and extend the event that was introduced in #460 (closed).
For reconfig done due to signal received, we could say root did it, although this would be somewhat imprecise. The owner of the process can send the signal as well. Kea is usually ran as root, but it doesn't have to. So perhaps better would be say signal in such cases?