agent: silence error on listener close (#97)
agent: silence error on listener close (#97)

As part of agent.Close, Close is called on the listener. This has
Accept return a "use of closed network connection" error, which is
expected. This error is logged but it would be nice if it wasn't.

Unfortunately, there's not a great way to check for this
error (https://golang.org/issues/4373) so use a best-effort
strings.Contains check.

google/gops