nanocoap_server: use zero-copy network buffer for parsing request

[benpicco]

Contribution description

Requests larger than CONFIG_NANOCOAP_SERVER_BUF_SIZE were previously silently ignored.
But we can do better: With sock_udp_recv_buf_aux() we don't need to copy the request into a separate buffer but can just use the buffer allocated by the network stack.

We still need CONFIG_NANOCOAP_SERVER_BUF_SIZE for the response as otherwise the size of the request packet would be the limit for the response packet. But this is now both in control of the server, a good server implementation can only use this for the header and keep the payload in a separate buffer.

The only problem is that several CoAP handler functions (e.g. _sha256_handler()) expect pkt->hdr to point to the response buffer and use pkt->hdr and buf interchangeably.

As a workaround, this also sets pkt->hdr = rbuf in coap_build_reply() to accommodate those handler functions.

Testing procedure

Issues/PRs references

[riot-ci]

Murdock results

✔️ PASSED

ec00a16 nanocoap: ensure pkt->hdr equals rbuf in coap_build_reply()

Success	Failures	Total	Runtime
10559	0	10560	11m:58s

Artifacts

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