Fix for html & docbook export of description list items

The problem is illustrated by the following example:

* Illustration of bug in html export
  - This has a space after the colons :: so will work in latex and html
  - This doesn't have a space after the colons ::so is an invalid
    description item according to the org manual.  Won't work in html
    or docbook.  Will nevertheless work in latex, provided /first/
    description item is valid.
  - Has a terminating space ::
    - So it works in both html and latex export!
    - Even though it's difficult to distinguish from the next example.
  - Lacks a terminating space ::
    - At present, *doesn't* work in html or docbook export, does in
      latex.  This is the case that the following patch fixes.

lisp/org-docbook.el

@@ -1382,7 +1382,7 @@ the alist of previous items."
       (string-match (concat "[ \t]*\\(\\S-+[ \t]*\\)"
 			    "\\(?:\\[@\\(?:start:\\)?\\([0-9]+\\|[a-zA-Z]\\)\\]\\)?"
 			    "\\(?:\\(\\[[ X-]\\]\\)[ \t]+\\)?"
-			    "\\(?:\\(.*\\)[ \t]+::[ \t]+\\)?"
+			    "\\(?:\\(.*\\)[ \t]+::\\(?:[ \t]+\\|$\\)\\)?"
 			    "\\(.*\\)")
 		    line)
       (let* ((checkbox (match-string 3 line))

lisp/org-html.el

@@ -2503,7 +2503,7 @@ the alist of previous items."
        (concat "[ \t]*\\(\\S-+[ \t]*\\)"
 	       "\\(?:\\[@\\(?:start:\\)?\\([0-9]+\\|[A-Za-z]\\)\\]\\)?"
 	       "\\(?:\\(\\[[ X-]\\]\\)[ \t]+\\)?"
-	       "\\(?:\\(.*\\)[ \t]+::[ \t]+\\)?"
+	       "\\(?:\\(.*\\)[ \t]+::\\(?:[ \t]+\\|$\\)\\)?"
 	       "\\(.*\\)") line)
       (let* ((checkbox (match-string 3 line))
 	     (desc-tag (or (match-string 4 line) "???"))