Add the ability to exclude dir-locals from subdirs. (Bug#8100)

Add the ability to exclude dir-locals from subdirs.  (Bug#8100)

* lisp/files.el (dir-locals-collect-variables):
Add the ability to exclude subdirectories.

* doc/emacs/custom.texi (Directory Variables):
Give an example of excluding subdirectories.

* doc/lispref/variables.texi (Directory Local Variables):
Mention `(subdirs . nil)' alist element.

* etc/NEWS: Mention this addition.

* lisp/dired-x.el (dired-omit-here-always): Add `(subdirs . nil)' to locals.