Auto merge of #1180 - xmclark:add-stdio-constants-windows, r=alexcrichton

add stdio constants for windows

A few useful constants for working with the stdio file descriptors.

I referenced the [msdn](https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/fileno?view=vs-2017) docs for the numbers, although I think 0,1,2 are standard  on most platforms for the stdio.