nl_langinfo: Add NL_LOCALE_NAME macro

nl_langinfo: Add NL_LOCALE_NAME macro

Provide an extension NL_LOCALE_NAME() macro, with semantics
matching glibc, which can be used as:
  nl_langinfo_l(NL_LOCALE_NAME(LC_MESSAGES), locale);
to get back the locale string that locale was originally
created with during newlocale(). This in turn allows a library
(such as gettext) to determine what thread-local locale settings
it has inherited from the main program without having to be told
what parameters were passed to newlocale(), for less overall
coupling between parts of the program.

gnulib is set up to use the extension:
https://lists.gnu.org/archive/html/bug-gnulib/2017-01/msg00129.html

* libc/include/langinfo.h (NL_LOCALE_NAME): New macro
* libc/locale/nl_langinfo.c (nl_langinfo_l): Expose locale names
of a locale_t's category components.

Signed-off-by: Eric Blake <eblake@redhat.com>