Use non-cryptographically-secure PRNG to generate a nonce for cookies.

Rationale: the nonce here is only used to make sure there is a low
probability of duplication, according to section B.2 of RFC7873.
It is only 32-bit, and even if an attacker knows the algorithm used
to generate nonces it won't, in any way, give him any platform to
attack the server as long as server secret used to sign the
(nonce, time) pair with HMAC-SHA1 is secure.

On the other hand, currently, each packet sent requires (unnecessarily)
a CS pseudo-random number which is ineffective.

lib/ns/client.c

@@ -1656,7 +1656,7 @@ ns_client_addopt(ns_client_t *client, dns_message_t *message,
 		isc_buffer_init(&buf, cookie, sizeof(cookie));
 		isc_stdtime_get(&now);
 
-		isc_nonce_buf(&nonce, sizeof(nonce));
+		isc_random_buf(&nonce, sizeof(nonce));
 
 		compute_cookie(client, now, nonce, client->sctx->secret, &buf);