dir-locals.el: Don't set variables for non-programming modes

dir-locals.el: Don't set variables for non-programming modes

This limits the style changes to modes inherited from prog-mode. The
main reason to do this is to avoid setting fill-column for people
using Emacs to edit commit messages because 78 characters is too many
to make it wrap properly in git log. Note that makefile-mode also
inherits from prog-mode so the fill column should continue to apply
there.

v2: Apply to all the .dir-locals.el files, not just the one in the
    root directory.

Acked-by: Michel Dänzer <michel.daenzer@amd.com>