ox-koma-letter: Add closing special heading

* ox-koma-letter.el (org-koma-letter-special-tags-in-letter):
  Add closing.
  (org-koma-letter-template): Potentially take closing value
  from closing spcical heading.
  (org-koma-letter--build-settings): Potentially take signature
  value from closing special heading.
  (org-koma-letter-closing):
  (org-koma-letter-signature): Update docstring.

Suggested-by: Suvayu Ali <fatkasuvayu+linux@gmail.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/99028>

contrib/lisp/ox-koma-letter.el

@@ -204,13 +204,27 @@ then the opening will be implicitly set as the headline title."
 
 (defcustom org-koma-letter-closing ""
   "Letter's closing, as a string.
-This option can also be set with the CLOSING keyword."
+This option can also be set with the CLOSING keyword.  Moreover,
+when:
+  (1) there's no CLOSING keyword or it is empty;
+  (2) `org-koma-letter-headline-is-opening-maybe' is non-nil;
+  (3) the letter contains a headline with the special
+      tag closing;
+then the opening will be set as the title of the closing special
+heading."
   :group 'org-export-koma-letter
   :type 'string)
 
 (defcustom org-koma-letter-signature ""
   "Signature, as a string.
-This option can also be set with the SIGNATURE keyword."
+This option can also be set with the SIGNATURE keyword.
+Moreover, when:
+  (1) there's no CLOSING keyword or it is empty;
+  (2) `org-koma-letter-headline-is-opening-maybe' is non-nil;
+  (3) the letter contains a headline with the special
+      tag closing;
+then the signature will be  set as the content of the
+closing special heading."
   :group 'org-export-koma-letter
   :type 'string)
 
@@ -358,7 +372,7 @@ e.g. \"title-subject:t\"."
     :group 'org-export-koma-letter
     :type 'boolean)
 
-(defconst org-koma-letter-special-tags-in-letter '(to from)
+(defconst org-koma-letter-special-tags-in-letter '(to from closing)
   "Header tags related to the letter itself.")
 
 (defconst org-koma-letter-special-tags-after-closing '(ps encl cc)
@@ -665,7 +679,17 @@ holding export options."
    ;; Letter body.
    contents
    ;; Closing.
-   (format "\n\\closing{%s}\n" (org-export-data (plist-get info :closing) info))
+   (format "\n\\closing{%s}\n"
+	   (org-export-data
+	    (or (org-string-nw-p (plist-get info :closing))
+		(when (plist-get info :with-headline-opening)
+		  (org-element-map (plist-get info :parse-tree) 'headline
+		    (lambda (head)
+		      (when (eq (org-koma-letter--special-tag head info)
+				'closing)
+			(org-element-property :title head)))
+		    info t)))
+	    info))
    (org-koma-letter--special-contents-as-macro
     (plist-get info :with-after-closing))
    ;; Letter end.
@@ -711,10 +735,20 @@ a communication channel."
           (format "\\KOMAoption{fromphone}{%s}\n"
                   (if (plist-get info :with-phone) "true" "false")))
      ;; Signature.
-     (let ((signature (plist-get info :signature)))
-       (and (org-string-nw-p signature)
-            (funcall check-scope 'signature)
-            (format "\\setkomavar{signature}{%s}\n" signature)))
+     (let* ((heading-val
+	     (and (plist-get info :with-headline-opening)
+		  (org-string-nw-p
+		   (org-trim
+		    (org-export-data
+		     (org-koma-letter--get-tagged-contents 'closing)
+		     info)))))
+	    (signature (org-string-nw-p (plist-get info :signature)))
+	    (signature-scope (funcall check-scope 'signature)))
+       (and (or (and signature signature-scope)
+		heading-val)
+	    (not (and (eq scope 'global) heading-val))
+	    (format "\\setkomavar{signature}{%s}\n"
+		    (if signature-scope signature heading-val))))
      ;; Back address.
      (and (funcall check-scope 'with-backaddress)
           (format "\\KOMAoption{backaddress}{%s}\n"