It turns out that in C++11, char16_t and char32_t are built-in types;
language keywords. Just fix this by putting traditional _*_T_DECLARED
blocks around the definitions. We'll just predefine these in
<sys/_types.h>.
This also opens up the possibility to define char16_t in other header
files, if ever needed (e.g. if we would gain a <ctype.h> for
char16_t/char32_t).
The <uchar.h> header, part of C11, adds a small number of utility
functions for 16/32-bit "universal" characters, which may or may not be
UTF-16/32. As our wchar_t is already ISO 10646, simply add light-weight
wrappers around wcrtomb() and mbrtowc().
While there, also add (non-yet-standard) _l functions, similar to the
ones we already have for the other locale-dependent functions.
Reviewed by: theraven
