ff00d66d69
This generalizes the existing code for answering CoAP pings into general message-layer responses. Such responses are now also sent as a reaction to CON responses, which can otherwise follow the same code path as existing other responses. As a side effect, issues that would crop up when responding to odd empty requests that have token length set are resolved. Contributes-To: https://github.com/RIOT-OS/RIOT/issues/14169