crnlib: correct alternate code to test if a path is a directory

[illwieckz]

The alternate code was testing for the file name in the current directory.

Fixes #63:

• macOS fails to detect if a path is a file or a directory on NFS if the file is not in current directory #63

[illwieckz]

One may open the folder with open then use fstatat on the file name to avoid concatenating a string, but we better assume doing an extra IO is slower than string concatenation.

[illwieckz]

Also an alternate code to use open on the parent dir to then use fdopendir instead of opendir to then reuse the fd with fstatat is not possible as the man page says:

After a successful call to fdopendir(), fd is used internally by the implemen‐
tation, and should not otherwise be used by the application.

So this may be the fastest code.

[slipher]

LGTM

[slipher]

I would call it known. The word "guess" implies you are making the answer and don't know it it's correct.

[illwieckz]

OK Done.