Hello,

COMMENT keyword is not always clearly defined in Org. Some parts
consider it is a regular keyword as treat it as such (e.g. `org-todo')
whereas some others see it as an additional keyword (e.g.,
`org-priority').

I think the latter makes more sense, and, as a consequence, Org Syntax
(http://orgmode.org/worg/dev/org-syntax.html) defines it that way. Much
like :ARCHIVE:, COMMENT provides an additional property to the headline,
without limiting it whatsoever.

In other words, the following should be valid:

  * TODO COMMENT Headline

and, according to Org Syntax, so should this:

  * TODO [#A] COMMENT Headline

Though, COMMENT keyword must come after any other keyword and priority,
if any. So the following is /not/ valid:

  * COMMENT TODO Headline

Thus, this patch

 - properly fontifies headlines with both a regular keyword and
   a COMMENT keyword,

 - fixes `org-toggle-comment' and `org-todo' to handle both COMMENT
   keyword and another one

 - adds some consistency to functions implementing their own COMMENT
   matching (e.g., with or without case-sensitivity).


WDYT?

-- 
Nicolas Goaziou