tools: next_unused_port: use IPPROTO_TCP explicitly

tools: next_unused_port: use IPPROTO_TCP explicitly

Otherwise perl tries to bind+listen on a UDP socket if the
TCP socket fails - which is a waste since we're looking for
TCP ports.
Additionall since UDP doesn't support listen(), perl will
return EOPNOTSUPP instead of, say, EADDRINUSE. (We don't
care about the error in this code though.)