gnrc_sock: warn about non-zero receive timeouts with sock_async

[miri64]

Contribution description

It took me nearly a day of debugging to find out that my event queue was broken due to the handler thread being stuck in a sock_udp_recv(). To prevent that in the future, issue a warning about non-zero-timeouts when the asynchronous callback is set.

Testing procedure

Apply the following patch:

diff --git a/tests/gnrc_sock_async_event/main.c b/tests/gnrc_sock_async_event/main.c
index d77e84c084..93bdc9a485 100644
--- a/tests/gnrc_sock_async_event/main.c
+++ b/tests/gnrc_sock_async_event/main.c
@@ -78,7 +78,7 @@ static void _recv_udp(sock_udp_t *sock, sock_async_flags_t flags, void *arg)
         sock_udp_ep_t remote;
         ssize_t res;
 
-        if ((res = sock_udp_recv(sock, _buffer, sizeof(_buffer), 0,
+        if ((res = sock_udp_recv(sock, _buffer, sizeof(_buffer), 20,
                                  &remote)) >= 0) {
             printf("Received UDP packet from [%s]:%u:\n",
                    ipv6_addr_to_str(_addr_str, (ipv6_addr_t *)remote.addr.ipv6,

Running make -C tests/gnrc_sock_async_event/ -j flash test for any platform should include the warning in the output

gnrc_sock: timeout != 0 with async callback set may lead to unexpected behavior.

It should not show up without the change.

Issues/PRs references

[miri64]

Added a comment stemming from offline discussions with @benpicco.