For symmetry with Cloneflags and it looks slightly weird because there
is syscall.Unshare method.

Change-Id: I3d710177ca8f27c05b344407f212cbbe3435094b
Reviewed-on: https://go-review.googlesource.com/23612Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>