* files.el (dir-locals-directory-cache): Change format to include

* files.el (dir-locals-directory-cache): Change format to include
the mtime of the directory-local variables file (Bug#2833).
(dir-locals-set-directory-class): New arg mtime.  Store it in
dir-locals-directory-cache.
(dir-locals-find-file): Check cache validity using the mtime.
(dir-locals-read-from-file): Save the file mtime in the cache.
(hack-dir-local-variables): Adapt to new
dir-locals-directory-cache entry format.