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
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