* libc/stdio/open_memstream.c: Add open_wmemstream to doumentation.

	* libc/stdio/open_memstream.c: Add open_wmemstream to doumentation.
	(struct memstream): Add wide element.
	Change saved to a union to take char and wchar_t values.
	(memwriter): Accommodate wide-oriented oeprations.
	(memseeker): Ditto.
	(memseeker64): Ditto.
	(memcloser): Ditto.
	(internal_open_memstream_r): New static function.  Take functionality
	from former _open_memstream_r and handle additional "wide" parameter.
	(_open_memstream_r): Just call internal_open_memstream_r with wide==-1
	from here.
	(_open_wmemstream_r): New function.
	(open_wmemstream): Ditto.
	* libc/include/wchar.h (open_wmemstream): Declare.
	(_open_wmemstream_r): Declare.