Ignore adapters with "Transport Name:   N/A" line in getmac
command output. This allows us to skip duplicate MAC addresses.

Fixes #19537.

Change-Id: I6b7be9d31322f963e02023c8f1037f6e9042b479
Reviewed-on: https://go-review.googlesource.com/39071Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Avelino <t@avelino.xxx>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>