nsapi_dns: Provide 2 IPv6-hosted default servers

Replace Comodo and OpenDNS IPv4 servers with Google and DNS.WATCH IPv6
servers, so IPv6-only devices (eg 6LoWPAN) have a default.

3 IPv4 resolvers should be plenty - existing code doesn't remember which
one last worked, so if early list entries were unreachable performance
would be consistently bad anyway. Replacing two entries avoids
increasing image size and RAM consumption.

On an IPv6-only or IPv4-only system, the sendto() for the wrong type of
address should fail immediately - change loop to move on to the next
server for any sendto() error.

Author: kjbracey

features/netsocket/nsapi_dns.cpp

@@ -31,11 +31,13 @@
 #define DNS_SERVERS_SIZE 5
 
 nsapi_addr_t dns_servers[DNS_SERVERS_SIZE] = {
-    {NSAPI_IPv4, {8, 8, 8, 8}},
-    {NSAPI_IPv4, {209, 244, 0, 3}},
-    {NSAPI_IPv4, {84, 200, 69, 80}},
-    {NSAPI_IPv4, {8, 26, 56, 26}},
-    {NSAPI_IPv4, {208, 67, 222, 222}},
+    {NSAPI_IPv4, {8, 8, 8, 8}},                             // Google
+    {NSAPI_IPv4, {209, 244, 0, 3}},                         // Level 3
+    {NSAPI_IPv4, {84, 200, 69, 80}},                        // DNS.WATCH
+    {NSAPI_IPv6, {0x20,0x01, 0x48,0x60, 0x48,0x60, 0,0,     // Google
+                  0,0, 0,0, 0,0, 0x88,0x88}},
+    {NSAPI_IPv6, {0x20,0x01, 0x16,0x08, 0,0x10, 0,0x25,     // DNS.WATCH
+                  0,0, 0,0, 0x1c,0x04, 0xb1,0x2f}},
 };
 
 // DNS server configuration
@@ -226,11 +228,9 @@ static int nsapi_dns_query_multiple(NetworkStack *stack, const char *host,
         dns_append_question(&question, host, version);
 
         err = socket.sendto(SocketAddress(dns_servers[i], 53), packet, DNS_BUFFER_SIZE);
-        if (err == NSAPI_ERROR_WOULD_BLOCK) {
+        // send may fail for various reasons, including wrong address type - move on
+        if (err < 0) {
             continue;
-        } else if (err < 0) {
-            result = err;
-            break;
         }
 
         // recv the response