The DNS Flag Day 2020 aims to remove the IP fragmentation problem from
the UDP DNS communication. In this commit, we implement the minimal
required changes by changing the defaults for `edns-udp-size`,
`max-udp-size` and `nocookie-udp-size` to `1232` (the value picked by
DNS Flag Day 2020).
(cherry picked from commit bb990030d3)
(cherry picked from commit df06773f9a73e4a4bb0b5e7f4bd8e40394e64032)
Ondřej Surý
c164c9a819