The Windows 7 getmac command may report the physical address of an adapter
as "Disabled" or "N/A". Handle these two cases to make the tests more
robust when building on Windows with manually disabled adapters or turned
off hardware.

Addresses issue #14130.

Change-Id: I0c2f8554b4b6810568e4e60ed53857599401f296
Reviewed-on: https://go-review.googlesource.com/19411Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>