coap_io_process: Fix 1ms busy loop and indefinite sleep

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.

examples/coap-server.c

@@ -1453,7 +1453,7 @@ main(int argc, char **argv) {
       }
       if (result > 0) {
         if (FD_ISSET(coap_fd, &readfds)) {
-          result = coap_io_process(ctx, COAP_RUN_NONBLOCK);
+          result = coap_io_process(ctx, COAP_IO_NO_WAIT);
         }
       }
     }

examples/contiki/server.c

@@ -216,7 +216,7 @@ PROCESS_THREAD(coap_server_process, ev, data)
     PROCESS_YIELD();
     if(ev == tcpip_event) {
       /* There is something to read on the endpoint */
-      coap_io_process(coap_context, COAP_RUN_BLOCK);
+      coap_io_process(coap_context, COAP_IO_WAIT);
     } else if (ev == PROCESS_EVENT_TIMER && etimer_expired(&dirty_timer)) {
       coap_resource_notify_observers(time_resource, NULL);
       etimer_reset(&dirty_timer);