* prolog.el: Fix indentation of empty line

* prolog.el: Fix indentation of empty line

* lisp/emacs-lisp/smie.el (smie-rules-function): Document new
`empty-line-token' element.
(smie-indent-empty-line): New function.
(smie-indent-functions): Add it.

* lisp/progmodes/prolog.el (prolog-smie-rules): Fix :list-intro behavior
and use the new `empty-line-token' element (bug#21526).
(prolog-mode-variables): Fix comment-start-skip setting to match
comment-start.

* test/indent/prolog.prolog: Add nested indentation tests.

* lisp/newcomment.el (comment-normalize-vars): Fix default value of
comment-start-skip not to misuse submatch 1.