xlocale.h is already a single-type micro-header, defining struct
__locale_struct and the typedefs __locale_t and locale_t.  This patch
brings it into the bits/types/ scheme: there are now
bits/types/__locale_t.h which defines only __locale_struct and
__locale_t, and bits/types/locale_t.h which defines locale_t as well
as the other two.  None of *our* headers need __locale_t.h, but it
appears to me that libstdc++ could make use of it.

There are a lot of external uses of xlocale.h, but all the uses I
checked had an autoconf test or equivalent for its existence.  It has
never been available from other C libraries, and it has always
contained a comment reading "This file is not standardized, don't rely
on it, it can go away without warning" so I think dropping it is
pretty safe.

I also took the opportunity to clean up comments in various public
header files that still talk about the *_l interfaces as though they
were completely nonstandard.  There are a few of them, notably the
strtoX_l and wcstoX_l families, that haven't been standardized, but
the bulk are in POSIX.1-2008.

        * locale/xlocale.h: Rename to...
	* locale/bits/types/__locale_t.h: ...here.  Adjust commentary.
	Only define struct __locale_struct and __locale_t, not locale_t.
        * locale/bits/types/locale_t.h: New file; define locale_t here.
        * locale/Makefile (headers): Update to match.

        * include/xlocale.h: Delete wrapper.
        * include/bits/types/__locale_t.h: New wrapper.
        * include/bits/types/locale_t.h: New wrapper.

        * ctype/ctype.h, include/printf.h, include/time.h
        * locale/langinfo.h, locale/locale.h, stdlib/monetary.h
        * stdlib/stdlib.h, string/string.h, string/strings.h, time/time.h
        * wcsmbs/wchar.h, wctype/wctype.h: Use bits/types/locale_t.h.
        Correct outdated comments regarding the standardization status of
        the functions that take locale_t arguments.

        * stdlib/strtod_l.c, stdlib/strtof_l.c, stdlib/strtol_l.c
        * stdlib/strtold_l.c, stdlib/strtoul_l.c, stdlib/strtoull_l.c
        * sysdeps/ieee754/ldbl-128ibm/strtold_l.c
        * sysdeps/ieee754/ldbl-64-128/strtold_l.c
        * wcsmbs/wcstod.c, wcsmbs/wcstod_l.c, wcsmbs/wcstof.c
        * wcsmbs/wcstof_l.c, wcsmbs/wcstold.c, wcsmbs/wcstold_l.c:
        Don't include xlocale.h. If necessary, include locale.h instead.

        * stdlib/strtold_l.c: Unconditionally include wchar.h.