Enhancement request: provide support for sys/uio.h

[asokolsky]

I have Linux code relying on the above header.
Thanks!

[robinrowe]

I haven't implemented readv/writev support because I have no application code that uses these calls. I implement POSIX calls for Windows as I encounter them in the wild in Linux code that I'm porting. If you ported these functions, feel free to send me a pull request.