The net.Listener interface specifies that on Close:
// Any blocked Accept operations will be unblocked and return errors.

Fixes golang/go#24458

Change-Id: I4a61a79db9579a40b536aa65c8077da87aa25156
Reviewed-on: https://go-review.googlesource.com/101535Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>