Add "kod" option to the ratelimit directive to respond with the KoD
RATE code to randomly selected requests exceeding the configured limit.
This complements the client support of KoD RATE. It's disabled by
default.
There can be only one KoD code in one response. If both NTS NAK and RATE
codes are triggered, drop the response. The KoD RATE code can be set in
an NTS-authenticated response.