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.

src/windows/gnu.rs

 pub const L_tmpnam: ::c_uint = 14;
 pub const TMP_MAX: ::c_uint = 0x7fff;
 
+// stdio file descriptor numbers
+pub const STDIN_FILENO: ::c_int = 0;
+pub const STDOUT_FILENO: ::c_int = 1;
+pub const STDERR_FILENO: ::c_int = 2;
+
 extern {
     pub fn strcasecmp(s1: *const ::c_char, s2: *const ::c_char) -> ::c_int;
     pub fn strncasecmp(s1: *const ::c_char, s2: *const ::c_char,