How to support local DNS


I have started a DNS server on port 5003 locally. However, `Builder.addDnsServer` doesn't support 127.0.0.1, nor does it support custom ports.

So, I used the following:

```java
String tun_dns = "172.19.0.2";
builder.addDnsServer(tun_dns);
builder.addRoute(tun_dns, 32);
```

But I'm not sure how to modify `udp.c to` correctly handle DNS requests.