voipchat: allow configuring server bind address from cli arguments

[madebr]

This allows you to bind the voipchat server to 127.0.0.1 or ::1.

This change is motivated by the following observations of current voipchat: When running voipchat --server, it was unclear to what socket the server was bound to. Also, the following combination failed for me: server command: ./voipchat --server clients command: ./voipchat localhost

Only after changing the client command to ./voipchat 127.0.0.1 could I get working voip.

This pr explicitly selects a binding address for the server, so it will never pass NULL to SDLNet_CreateDatagramSocket when creating a server.

Perhaps https://github.com/libsdl-org/SDL_net/pull/85 is related.