[root] reports whether the current effective user ID is 0.

Updates #29127

Change-Id: I9ef42f1271ea669689011e7ceff4d918c0cecb6b
Reviewed-on: https://go-review.googlesource.com/c/153637
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>