gcoap_req_send returns 0 if it was unable to send the CoAP request. CoRD
did not include that case in the return code checks. This changes CoRD
to include it and drop the registration if CoAP could not send the
request. The old behaviour made the CoRD thread lock up.
This is an API change in the latter, which would typically now take an
extra argument GCOAP_SOCKET_TYPE_UNDEF.
Follow-Up-For: https://github.com/RIOT-OS/RIOT/pull/16688
Replace direct accesses to sched_active_thread and sched_active_pid with
the helper functions thread_getpid() and thread_get_active(). This serves
two purposes:
1. It makes accidental writes to those variable from outside core less likely.
2. Casting off the volatile qualifier is now well contained to those two
functions
