Add UV::S_IFDIR + UV::S_IFREG constants

These constants allow differentiation between regular files (S_IFREG)
and directories (S_IFDIR) when used for bitwise AND operations against
the "mode" value returned from uv_fs_stat/fstat/lstat().

Example:

    uv_fs_open(uv_default_loop(), __FILE__, UV::O_RDONLY, 0, function($r){
        uv_fs_fstat(uv_default_loop(), $r, function($result, $da){
            echo "is directory: ", ($da['mode'] & UV::S_IFDIR ? 'yes' : 'no'), "\n";
            echo "if file: ", ($da['mode'] & UV::S_IFREG ? 'yes' : 'no'), "\n";
        });
    });
    uv_run();