Assume a default interface if there's only one non-loopback interface

[JonathanLennox]

In the common case where a machine has only a single non-loopback interface, having to type the interface name to the tcconfig commands seems unfortunate, especially because it makes them non-portable across systems. There should be a default.

(Perhaps even in the multi-interface cases, have it default to the interface the default route is on?)