fs: ifx uv_fs_readdir_next reported types

Some systems (hello SunOS!) don't have thr d_type field on struct
dirent, so mark them as UV_DIRENT_UNKNOWN, also mark anything not a
directory or a file as unknown.

Jenkins, give me some good news!
After a second thought, I don't really enjoy this line. Could we do `ifdef UV__DT_FILE` here?

It is very painful to polyfill this, if the most of the file types are unknowns. In fact, the most useful information is dir/not-dir here, and with your patch we are loosing it.