locale: Implement getlocalename_l.

Specified in POSIX-2024.

GNUlib currently has a nasty Haiku-specific hack to implement
equivalent functionality; after this it should be able to use
this function instead.

src/system/libroot/posix/locale/locale_t.cpp

@@ -260,6 +260,17 @@ uselocale(locale_t newLoc)
 }
 
 
+extern "C" const char*
+getlocalename_l(int category, locale_t l)
+{
+	LocaleBackend* backend = (l == LC_GLOBAL_LOCALE) ?
+		gGlobalLocaleBackend : (LocaleBackend*)((LocaleBackendData*)l)->backend;
+	if (backend == NULL)
+		return "POSIX";
+	return backend->SetLocale(category, NULL);
+}
+
+
 extern "C" locale_t
 __current_locale_t()
 {

src/system/libroot/posix/locale/setlocale.cpp

@@ -28,13 +28,8 @@ setlocale(int category, const char* locale)
 	if (category < 0 || category > LC_LAST)
 		return NULL;
 
-	if (locale == NULL) {
-		// query the locale of the given category
-		if (gGlobalLocaleBackend != NULL)
-			return const_cast<char*>(gGlobalLocaleBackend->SetLocale(category, NULL));
-		else
-			return const_cast<char*>("POSIX");
-	}
+	if (locale == NULL)
+		return const_cast<char*>(getlocalename_l(category, LC_GLOBAL_LOCALE));
 
 	// we may have to invoke SetLocale once for each category, so we use an
 	// array to collect the locale per category