fix behavior of printf with alt-form octal, zero precision, zero value

in this case there are two conflicting rules in play: that an explicit
precision of zero with the value zero produces no output, and that the
'#' modifier for octal increases the precision sufficiently to yield a
leading zero. ISO C (7.19.6.1 paragraph 6 in C99+TC3) includes a
parenthetical remark to clarify that the precision-increasing behavior
takes precedence, but the corresponding text in POSIX off of which I
based the implementation is missing this remark.

this issue was covered in WG14 DR#151.
implement a private state for the uchar.h functions

The C standard is imperative on that:

  7.28.1 ... If ps is a null pointer, each function uses its own internal
  mbstate_t object instead, which is initialized at program startup to
  the initial conversion state;

and these functions are also not supposed to implicitly use the state of
the wchar.h functions:

  7.29.6.3 ... The implementation behaves as if no library function calls
  these functions with a null pointer for ps.

Previously this resulted in two bugs.

 - The functions c16rtomb and mbrtoc16 would crash when called with ps
   set to null.

 - The function mbrtoc32 used the private state of mbrtowc, which it
   is not allowed to do.