Add authoritative support
We tried to deploy Kea in our campus as a replacement for ISC dhcpd, but had issues due to the lack of authoritative support in Kea. Windows machines in particular stubbornly tried to request addresses from other networks, which was a show stopper in our campus WiFi, where users use their own devices. The IETF NOC had probably the same issue.
This PR adds a new boolean authoritative setting to the Dhcp4 config as a global, shared-network, and subnet configuration scopes and uses this setting to influence Kea's behaviour with regards to unknown clients. The behavioural change in the allocation path is actually quite trivial. I hope, I haven't overlooked something.
I've split the various parts of this PR into different commits for easier integration. The files changed by Bison/Flex are in a separate commit in particular, because I have newer versions than you of these tools on my development machine.
This PR addresses trac issue 5184 and GitLab issue 66. It has been opened on GitHub, but I opened it here too after request from @tomek.