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!
Let me check what we have on the libcs supporting dirent types. IMHO doing `stat` is overkill. I'd rather say unknown since we don't actually know and let the user stat if he wants to.