As suggested by @colinmkeith the UDP and DNS proxies should listen on different
ports otherwise the DNS proxy can get traffic intended to the UDP proxy (or
vice-versa) and handle it incorrectly as reported in #178.

At first sight it seems that we had the code in place to try another port if
the one we are binding is already bound, however, with UDP and REUSEADDR the
OS will not refuse to bind two sockets to the same socket address, so both
the UDP proxy and DNS proxy were being bound to the same pair.