rmdir -p bug

[Wolfram Kleff]

> rmdir --version
rmdir (fileutils) 4.1

rmdir -p should not try to rmdir ".":
Example: create an empty dir:
> mkdir a
Try to remove the dir with leading "./"
> rmdir --ignore-fail-on-non-empty -p ./a/
rmdir: `.': Invalid argument
Exit 1

rmdir should neither write "mdir: `.': Invalid argument", nor
should it exit with a failure (Exit 1)