test: Don't use ifconfig, as it's less available on linux

Use the 'ip' command to test for the presence of an interface
instead.

Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>
* Fixed the path to the IP binary to be more compatible
ws: Track when we sent an EOF to each agent

This prevents races while shutting down, and is a patch
that was accidentally not pushed in the earlier multi-agent
work.

Closes #443