Still running multihomed, clearly need to do at least some tests in a
channel-specific manner. The easy thing to start with is probe_ping,
since it has a -S option for source address, which should do the right
thing when combined with our iprules. However, since this is testing
the interface, not the remote resource, it isn't an option to
probe_ping, but a new probe_interface or something like that.
Later versions should probably do similar tests with TCP.