illumos gate

To properly implement support for open_wmemstream() we need to be able to write embedded zero byte wide characters. Unfortunately, the interfaces that we have today have no way of indicating how many bytes was actually consumed when the zero is translated. This makes it very difficult to correctly write a program here as there is no guarantee a locale uses a single byte and in fact looking through many of the locales, it is not always the case that there is a single byte. To rectify this, I'd like to add a private function to libc which does a variant of mbrtowc() which understands this. We could consider making this public, but to do so we should work with the broader open source communities to agree on a name for this.

To summarize, this allows a variant of mbrtowc() to always indicate the number of bytes consumed, even if it results in a wide-zero which would normally only indicate 'zero'.