os.DirFs(pth) seems to have a fatal design flaw: It invites the user to hardcode either a UNIX (e.g., /) or Windows (e.g., C:) root directory name.

Please implement a 0-ary constructor for filefs that automatically selects an appropriate root directory name in an operating system independent way.

Note that Cygwin environments can reasonably follow UNIX conventions (/); It's not the executable platform so much as the runtime environment.