Fix waitid namespace (bug 21561)

[Zack Weinberg]

On Fri, Jun 9, 2017 at 11:07 AM, Joseph Myers <joseph@codesourcery.com> wrote:
> In sys/wait.h, waitid and associated constants and types are UX-shaded
> in XPG4.2 (so not in XPG4), and XSI-shaded in POSIX before 2008, so
> should be appropriately conditional in the headers.  This patch fixes
> the conditionals accordingly.  (WCONTINUED is actually still
> XSI-shaded in POSIX.1:2008, but W* is also reserved there without
> XSI-shading, so nothing special needs to be done about the
> conditionals on WCONTINUED to conform to POSIX.1:2008 namespace
> rules.)

LGTM.

zw