Contribution description

When module nanocoap_server_separate is not used, the functions to send separate responses are still provided, just in a broken version: They will send the separate replies from a different endpoint than the request was received at (even on machines with only one IP address, as also the source port is randomized).

This changes the behavior to only provide the functions for separate response when the do work, so that others will detect an invalid configuration at compile time rather than at run time.

The documentation is duly updated.

Testing procedure

$ make BOARD=same54-xpro flash term -C examples/nanocoap_server
[...]
2024-12-12 10:30:46,488 # {"IPv6 addresses": ["fe80::fec2:3dff:fe0b:cf49"]}
2024-12-12 10:30:49,421 # _separate_handler(): send ACK, schedule response
2024-12-12 10:30:50,425 # _separate_handler(): send delayed response

$ coap-client-notls -p 1234 -a 'fe80::183:ade0:3cd0:d5b%enp1s0f0' -m get 'coap://[fe80::fec2:3dff:fe0b:cf49]/separate'
This is a delayed response.

Issues/PRs references

None