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/cygwin/release/3.1.0

@@ -16,6 +16,9 @@ What changed:
 - If a SA_SIGINFO signal handler changes the ucontext_t pointed to by
   the third parameter, follow it after returning from the handler.
 
+- Eliminate a header file name collision with <X11/XLocale.h> on case
+  insensitive filesystems by reverting <xlocale.h> back to <sys/_locale.h>.
+
 
 Bug Fixes
 ---------