gcoap: Process CON responses
gcoap: Process CON responses

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

OTAkeys/RIOT,jasonatran/RIOT,jasonatran/RIOT,miri64/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,miri64/RIOT,kaspar030/RIOT,authmillenon/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,kYc0o/RIOT,jasonatran/RIOT,OTAkeys/RIOT,ant9000/RIOT,kYc0o/RIOT,authmillenon/RIOT,authmillenon/RIOT,miri64/RIOT,kaspar030/RIOT,OTAkeys/RIOT,kYc0o/RIOT,OlegHahm/RIOT,jasonatran/RIOT,miri64/RIOT,kYc0o/RIOT,kaspar030/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kYc0o/RIOT,authmillenon/RIOT,OTAkeys/RIOT,miri64/RIOT,OlegHahm/RIOT,ant9000/RIOT,ant9000/RIOT,ant9000/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,kaspar030/RIOT,OlegHahm/RIOT,ant9000/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,authmillenon/RIOT