* lisp/textmodes/page.el (page--what-page): Adjust for 1st
line on page, and use 'count-lines' again. (Bug#68215)
* test/lisp/textmodes/page-tests.el (page-tests-what-page):
Update test.
* lisp/textmodes/page.el (page--count-lines-page): New function
extracted from `count-lines-page'.
(count-lines-page): Extract main logic into `page--count-lines-page'.
(page--what-page); New function extracted from `what-page'.
(what-page): Extract main logic into `page--what-page'.
* test/lisp/textmodes/page-tests.el (page-tests-count-lines-page)
(page-tests-what-page): New tests for `page--count-lines-page' and
`page--what-page'. (Bug#36009)
* lisp/textmodes/page.el: Use lexical-binding.
(forward-page): Replace `(if x nil y)' with `(unless x y)'.
* test/lisp/textmodes/page-tests.el: New file with tests for page.el.