Allow piped stdout/stderr use uv_tty_t

There are issues with reading stdin when it is actually attached to a pipe, but
I have run into no problems in writing to stdout/stderr when they are attached
to pipes.

src/libstd/io/native/file.rs

@@ -167,6 +167,7 @@ impl rtio::RtioTTY for FileDesc {
     fn get_winsize(&mut self) -> Result<(int, int), IoError> {
         Err(super::unimpl())
     }
+    fn isatty(&self) -> bool { false }
 }
 
 impl Drop for FileDesc {