rmdir: diagnose non following of symlinks with trailing slash

rmdir: diagnose non following of symlinks with trailing slash

GNU/Linux is unusual here in that rmdir("symlink/") returns ENOTDIR,
whereas Solaris and FreeBSD at least, will follow the symlink
and remove the target directory.  We don't make the behavior
on Linux kernels consistent, but at least clarify
the confusing error message.

* src/rmdir (main): Output a specific error message for the above case.
(remove_parents): In the error message, don't assume intermediate paths
are directories, as they could be symlinks.
* tests/rmdir/symlink-errors.sh: Add a new test.
* tests/local.mk: Reference the new test.
* NEWS: Mention the improvement.