[O] Minor fix for org-attach-dir

[Gustav Wikström]

For full transparency: An issue was found in org-brain [1] due to a
change in functionality of org-attach-dir in commit ae9cd4370.

I've given it some thought and figured that particular change in
functionality wasn't really needed and not obviously correct either.
So org-attach-dir will now check the filesystem (again) to see if the
attachment folder from DIR or ID property exist or not. The check can
be skipped with a new optional flag.

See commit 4b7eda1a2 for the fix.

[1] https://github.com/Kungsgeten/org-brain/pull/203

Regards
Gustav