vasnprintf: Work around two grouping bugs on native Windows. * m4/vasnprintf.m4 (gl_PREREQ_VASNPRINTF_FLAG_GROUPING): On native Windows, define NEED_PRINTF_FLAG_GROUPING. * lib/vasnprintf.c (localeconv): Undefine. (thousands_separator_char, thousands_separator_wchar): On native Windows, use localeconv(). * tests/test-vasnprintf-posix2.c (main): On native Windows, expect 3 digits for the exponent. * tests/test-vasnwprintf-posix2.c (main): Likewise. * doc/posix-functions/fprintf.texi: Mention the mingw and MSVC bugs. * doc/posix-functions/vfprintf.texi: Likewise. * doc/posix-functions/printf.texi: Likewise. * doc/posix-functions/vprintf.texi: Likewise. * doc/posix-functions/sprintf.texi: Likewise. * doc/posix-functions/vsprintf.texi: Likewise. * doc/posix-functions/snprintf.texi: Likewise. * doc/posix-functions/vsnprintf.texi: Likewise. * doc/posix-functions/dprintf.texi: Likewise. * doc/posix-functions/vdprintf.texi: Likewise. * doc/posix-functions/fwprintf.texi: Likewise. * doc/posix-functions/vfwprintf.texi: Likewise. * doc/posix-functions/wprintf.texi: Likewise. * doc/posix-functions/vwprintf.texi: Likewise. * doc/posix-functions/swprintf.texi: Likewise. * doc/posix-functions/vswprintf.texi: Likewise. * doc/posix-functions/asprintf.texi: Likewise. * doc/posix-functions/vasprintf.texi: Likewise. * doc/glibc-functions/obstack_printf.texi: Likewise. * doc/glibc-functions/obstack_vprintf.texi: Likewise.