gcoap: gcoap_req_send and related should return negative for errors

[bergzand]

Description

gcoap_req_send currently returns non-zero positive on success and either zero or negative for failed transfers. This is error prone for users, even internally (see #19383).

Steps to reproduce the issue

Read the documentation

Expected results

Clear values for errors

Actual results

Both negative and zero are used to signal errors

[chrysn]

This also affects gcoap_req_send_tl (which AIU is stuck in an API-compatibility-change deprecation cycle that hasn't been completed); are there any other function with weird zero results?

[bergzand]

This also affects gcoap_req_send_tl (which AIU is stuck in an API-compatibility-change deprecation cycle that hasn't been completed); are there any other function with weird zero results?

gcoap_req_send_tl() is the cause behind this as gcoap_req_send() calls it with a specific socket type.

The other one I've found is not necessarily weird, but gcoap_obs_send squashes all negative error codes from the internal _tl_send() to zero. That's the only one I've found in the gcoap code.