Make listening on UDP port optional

I am planning to run pushd on a somewhat public server using IP filtering.

TCP itself is somewhat vulnerable to MITM attacks, therefore source IP is not a very reliable authorization. However, I do not consider TCP to be an issue at this point. The UDP handler of pushd on the other hand is receive-only, which means the server never even tries to contact the sender to prove if the packet was valid or not.

Therefore for security reasons I would like to disable UDP handling for now, it doesn't bring any extra benefits to me because all the functionality is available through the REST API as well. I might consider some simple HTTP authentication scheme later on, but for now this should be enough to get things running.