Replace COAP_RUN_BLOCK with COAP_IO_WAIT and COAP_RUN_NOBLOCK with
COAP_IO_NO_WAIT for naming clarity.
Instead of COAP_IO_NO_WAIT being 1, make it -1 to fix 1ms busy loop.
Handle the case when coap_io_prepare_io returns 0 to prevent indefinite
select() wait.
include/coap2/net.h:
Make the timeout_ms parameter uint32_t in coap_io_process*() functions to
make sure special case of COAP_IO_NO_BLOCK is correctly handled on different
architectures.
Update COAP_IO_NO_BLOCK to -1.
src/coap_io.c:
Fix the 1ms busy loop and potential infinite wait in the non epoll case.
man/coap_io.txt.in:
Update documentation to reflect the changes.
examples/coap-server.c:
examples/contiki/server.c:
Update to use COAP_IO WAIT and COAP_IO_NO_WAIT as appropriate.