fix not supported file system cases

[DaniilSNikulin]

from documentation for function readdir:

Currently, only some filesystems (among them: Btrfs, ext2, ext3,
and ext4) have full support for returning the file type in d_type.
All applications must properly handle a return DT_UNKNOWN

To avoid problems with not supported file system, is_dir flag additionally computed via function stat, which don't have same constraints.

In my case, DT_UNKNOWN was given from readdir, if sub-folder is mounted via sshfs.

[simonfuhrmann]

Thanks!