Conform to elisp header conventions

Headings should begin with at least three semicolons.  Top-level
sections are prefixed with three, subsections with four and so on.

From `(elisp)Comment Tips':

`;;;'
     Comments that start with three semicolons, `;;;', should start at
     the left margin.  We use them for comments which should be
     considered a "heading" by Outline minor mode.  By default,
     comments starting with at least three semicolons (followed by a
     single space and a non-whitespace character) are considered
     headings, comments starting with two or fewer are not.
     Historically, triple-semicolon comments have also been used for
     commenting out lines within a function, but this use is
     discouraged.

     When commenting out entire functions, use two semicolons.

`;;;;'
     Comments that start with four semicolons, `;;;;', should be aligned
     to the left margin and are used for headings of major sections of a
     program.  For example:

The trailing colons are redundant, except after "Commentary" and "Code",
where `lisp-mnt.el' unfortunately expects them.  "Code:" is necessary
because, in some contexts, it also serves as the terminator for
"Commentary:".

Do not nest other code sections inside "Code:", departing a bit from
what is said about ";;;;" above.  Instead define other code sections as
top sections too, and treat "Code:" only as the first code section and
use it for "dependencies and other setup".  Add a few additional
sections to avoid putting to much non-setup code inside "Code:".