std: Change how EBADF is handled in sys

This commit removes the reexport of `EBADF_ERR` as a constant from
libstd's portability facade, instead opting for a platform-specific
function that specifically queries an `io::Error`. Not all platforms may
have a constant for this, so it makes the intent a little more clear
that a code need not be supplied, just an answer to a query.

src/libstd/io/stdio.rs

@@ -121,10 +121,8 @@ impl<R: io::Read> io::Read for Maybe<R> {
 }
 
 fn handle_ebadf<T>(r: io::Result<T>, default: T) -> io::Result<T> {
-    use sys::stdio::EBADF_ERR;
-
     match r {
-        Err(ref e) if e.raw_os_error() == Some(EBADF_ERR) => Ok(default),
+        Err(ref e) if stdio::is_ebadf(e) => Ok(default),
         r => r
     }
 }