* 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.