Proper MB_CUR_MAX.

Previously this was hard coded to 4. This is only the case for UTF-8
locales.

As a side effect, this properly reports C.UTF-8 as the default locale
instead of C.

Change-Id: I7c73cc8fe6ffac61d211cd5f75287e36de06f4fc

tests/stdio_test.cpp

@@ -427,6 +427,9 @@ TEST(stdio, snprintf_negative_zero_5084292) {
 }
 
 TEST(stdio, snprintf_utf8_15439554) {
+  locale_t cloc = newlocale(LC_ALL, "C.UTF-8", 0);
+  uselocale(cloc);
+
   // http://b/15439554
   char buf[BUFSIZ];
 
@@ -442,6 +445,8 @@ TEST(stdio, snprintf_utf8_15439554) {
   // 4-byte character.
   snprintf(buf, sizeof(buf), "%d\xf0\xa4\xad\xa2%d", 1, 2);
   EXPECT_STREQ("1𤭢2", buf);
+
+  freelocale(cloc);
 }
 
 TEST(stdio, fprintf_failures_7229520) {