libintl: Ensure the *printf function overrides are POSIX compatible.
Reported by Eli Zaretskii <eliz@gnu.org>.
* gettext-runtime/intl/printf.c (USE_REPLACEMENT_CODE_ALWAYS): New macro.
(libintl_vfprintf, libintl_vsprintf, libintl_vsnprintf, libintl_vfwprintf,
libintl_vswprintf): Use it.
* NEWS: Mention the change.
