This adds the NETNSA constants from net_namespace.h, used for creating
and querying network namespace IDs.

Fixes golang/go#26330
Change-Id: I786c4c4771be3eddc4fdb830a52e128aa1a2b9fd
Reviewed-on: https://go-review.googlesource.com/124436
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>