Implement a more generic constructor for rootfs

[mcandre]

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.

[hairyhenderson]

Thanks for this suggestion - can you give me a concrete example of how you imagine this might look? I'm not sure I'm totally following what the concern is...

[github-actions]

This issue is stale because it has been open for 60 days with no
activity. If it is no longer relevant or necessary, please close it.
Given no action, it will be closed in 14 days.

If it's still relevant, one of the following will remove the stale
marking:

• A maintainer can add this issue to a milestone to indicate that
  it's been accepted and will be worked on
• A maintainer can remove the stale label
• Anyone can post an update or other comment