Improve consistency of outline headings in dired libraries

Improve consistency of outline headings in dired libraries

* lisp/dired-aux.el: Improve consistency of outline headings.
* lisp/dired-x.el: Improve consistency of outline headings.
* lisp/dired.el: Improve consistency of outline headings.

- Use three semicolons at beginning of headings because that already
  is the dominant number of semicolons for headings in these files.
- Prefix each heading with a ^L instead of randomly omitting it in
  front of some.
- Always prefix the line with the ^L with a completely empty line.
- Begin headings with a capital letter.
- Do not update complete headings.
- Do not end headings with a period.
- Remove a handful of section end markers.
- Address further inconsistencies.