Concurrent calls to Interface methods on autodisc could return a "not
discovered" error if the discovery did not finish before the call.
autodisc.wait expected the done channel to carry the found Interface
but it was closed instead.
The fix is to use sync.Once for now, which is easier to get right.
And there is a test. Finally.
This will have to change again when we introduce re-discovery.
I have verified that UPnP and NAT-PMP work against an older version of
the MiniUPnP daemon running on pfSense. This code is kind of hard to
test automatically.
