Add printf wchar support for %lc (%C) and %ls (%S).

Add printf wchar support for %lc (%C) and %ls (%S).
Require printf format strings to be valid multibyte strings beginning and
  ending in their initial shift state, as per the stds.

Fixed a bug in _wchar_wcsntoutf8s().  Don't store wcs position if dst is NULL.
Also, introduce an awful hack into _wchar_wcsntoutf8s() and wcsrtombs() in
  order to support %ls in printf.  See comments below for details.
Change behaviour of wc<->mb functions when in the C locale.  Now they do
  a 1-1 map for the range 0x80-UCHAR_MAX.  This is for backwards compatibility
  and consistency with the stds requirements that a printf format string by
  a valid multibyte string beginning and ending in it's initial shift state.