Addresses issue 2802: 'n' formatting for integers.

Adds 'n' as a format specifier for integers, to mirror the same
specifier which is already available for floats.  'n' is the same as
'd', but inserts the current locale-specific thousands grouping.

I added this as a stringlib function, but it's only used by str type,
not unicode.  This is because of an implementation detail in
unicode.format(), which does its own str->unicode conversion.  But the
unicode version will be needed in 3.0, and it may be needed by other
code eventually in 2.6 (maybe decimal?), so I left it as a stringlib
implementation.  As long as the unicode version isn't instantiated,
there's no overhead for this.

Include/stringobject.h

@@ -176,7 +176,17 @@ PyAPI_FUNC(int) PyString_AsStringAndSize(
 				   (only possible for 0-terminated
 				   strings) */
     );
-    
+
+/* Using the current locale, insert the thousands grouping
+   into the string pointed to by buffer.  For the argument descriptions,
+   see Objects/stringlib/localeutil.h */
+
+PyAPI_FUNC(int) _PyString_InsertThousandsGrouping(char *buffer,
+						  Py_ssize_t len,
+						  char *plast,
+						  Py_ssize_t buf_size,
+						  Py_ssize_t *count,
+						  int append_zero_char);
 
 #ifdef __cplusplus
 }