Rename <xlocale.h> back to <sys/_locale.h>

libX11 provides <X11/Xlocale.h>.  The build of libX11 itself adds
include/X11 to the compiler's include path.  This results in a name
collision with /usr/include/xlocale.h on case-insensitive filesystems.

Commit 90e35b1eb3 renamed sys/_locale.h to xlocale.h in March 2017 under
the assumption that we should provide the locale_t type in the same file
as on Linux, FreeBSD, and Darwin.

A few weeks later (June 2017), glibc removed the xlocale.h file in favor
of bits/types/locale_t.h, which shouldn't be included directly anyway.

For reference and the reasoning, see
https://sourceware.org/git/?p=glibc.git;a=commit;h=f0be25b6336d

Given the above, revert 90e35b1eb3 and
fix additional usage of xlocale.h.

winsup/doc/new-features.xml

@@ -8,6 +8,12 @@
 
 <itemizedlist mark="bullet">
 
+<listitem><para>
+Eliminate a header file name collision with &lt;X11/XLocale.h&gt; on
+case insensitive filesystems by reverting &lt;xlocale.h&gt; back to
+&lt;sys/_locale.h&gt;.
+</para></listitem>
+
 <listitem><para>
 FIFOs can now be opened multiple times for writing.
 </para></listitem>