Add auto-mode-alist functionality to .dir-locals.el

Add auto-mode-alist functionality to .dir-locals.el

* doc/emacs/custom.texi (Directory Variables): Document
auto-mode-alist in .dir-locals.el (Bug#18721)
* doc/emacs/modes.texi (Choosing Modes): Update.
* lisp/files.el (set-auto-mode--apply-alist): New function,
from set-auto-mode.
(set-auto-mode): Check directory locals for auto-mode-alist.
(dir-locals-collect-variables): Add "predicate" parameter.
(hack-dir-local--get-variables): New function, from
hack-dir-local-variables.
(hack-dir-local-variables): Call hack-dir-local--get-variables.
* test/lisp/files-resources/.dir-locals.el: New file.
* test/lisp/files-resources/whatever.quux: New file.
* test/lisp/files-tests.el (files-tests-data-dir): New variable.
(files-test-dir-locals-auto-mode-alist): New test.