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!
Actually, I'd even try to polyfill it on solaris. @tjfontaine are there any way, except `stat` to get what we need here?