Don't parse %s format argument as multibyte string

(cherry picked from commit 715a900c)

2012-09-28  Andreas Schwab  <schwab@linux-m68k.org>

	[BZ #6530]
	* stdio-common/vfprintf.c (process_string_arg): Revert
	2000-07-22 change.

2011-09-28  Jonathan Nieder  <jrnieder@gmail.com>

	* stdio-common/Makefile (tst-sprintf-ENV): Set environment
	for testcase.
	* stdio-common/tst-sprintf.c: Include <locale.h>
	(main): Test sprintf's handling of incomplete multibyte
	characters.