Gcoap and the "All CoAP Nodes" Multicast Group

Hello everyone,

Working with gcoap, I noticed that it does not seem to join the “All CoAP Nodes” IPv6 multicast group (see RFC 7252, section 12.8) by default. Is this a feature that should be added to gcoap (or nanocoap)? If so, I would try to open a PR for this in the next couple of days :slight_smile: