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>

newlib/libc/include/langinfo.h

@@ -304,7 +304,7 @@ enum __nl_item
   _NL_COLLATE_CODESET,
 
   /* This MUST be the last entry since it's used to check for an array
-     index in nl_langinfo(). */
+     index in nl_langinfo(). It also must not exceed _NL_LOCALE_NAME_BASE. */
   _NL_LOCALE_EXTENDED_LAST_ENTRY
 
 #endif /* __HAVE_LOCALE_INFO_EXTENDED__ */
@@ -312,6 +312,14 @@ enum __nl_item
 
 };
 
+/* As an extension, nl_langinfo can retrive the name of a locale
+   category, with this mapping from setlocale() category (other than
+   LC_ALL) to nl_item. */
+#define _NL_LOCALE_NAME_BASE 100000
+#if __GNU_VISIBLE
+#define NL_LOCALE_NAME(category) (_NL_LOCALE_NAME_BASE + (category))
+#endif
+
 __BEGIN_DECLS
 char	*nl_langinfo (nl_item);
 #if __POSIX_VISIBLE >= 200809

newlib/libc/locale/nl_langinfo.c

@@ -24,6 +24,8 @@
  * SUCH DAMAGE.
  */
 
+#define _GNU_SOURCE
+
 #include <sys/cdefs.h>
 
 #include <locale.h>
@@ -368,6 +370,13 @@ do_codeset:
 		break;
 #endif
 	default:
+		/* Relies on the fact that LC_ALL is 0, and all other
+		   LC_ constants are in ascending order. */
+		if (item > NL_LOCALE_NAME(LC_ALL)
+		    && item < NL_LOCALE_NAME(_LC_LAST)) {
+			return locale->categories[item
+						  - NL_LOCALE_NAME(LC_ALL)];
+		}
 #ifdef __HAVE_LOCALE_INFO_EXTENDED__
 		if (item > _NL_LOCALE_EXTENDED_FIRST_ENTRY
 		    && item < _NL_LOCALE_EXTENDED_LAST_ENTRY) {