Add Stdio.File()->cd() on platforms that have fchdir(2)
Example:
Stdio.File dir_fd = Stdio.File(some_dir, "r");
dir_fd->cd(); // cd(some_dir);
dir_fd->cd("foo"); // cd(combine_path(some_dir, "foo"));
Example:
Stdio.File dir_fd = Stdio.File(some_dir, "r");
dir_fd->cd(); // cd(some_dir);
dir_fd->cd("foo"); // cd(combine_path(some_dir, "foo"));