make dir-locals work again with emacs 26.1

make dir-locals work again with emacs 26.1

After upgrading to emacs-26.1-1.fc28.x86_64 I noticed that our
.dir-locals.el files weren't honoured anymore (specifically the fill
column variable is not correctly set for c-mode files). I finally
tracked this down to the order in which items are listed in
.dir-locals.el: if the "nil" one is listed last everything works,
otherwise, it's the one that is applied instead of the c-mode one.

This patch simply swaps the entries, and puts the "nil" one last. My
emacs lisp fu is a bit too limited to understand the full impact for
this, and why emacs 26.1 changed behaviour in this regard, but from an
outsider's view the order shouldn't negatively affect things otherwise,
hence this patch.