Add a variable to pass additional options to rcvstore

* lisp/gnus/gnus-mh.el (gnus-rcvstore-options): New variable.
(gnus-summary-save-in-folder): Include gnus-rcvstore-options in
the arguments that are passed to rcvstore.

cf. <nntp://news.gmane.org/gmane.emacs.gnus.general/87263>
and followups, i.e., ding mailing list.

etc/GNUS-NEWS

@@ -266,6 +266,9 @@ on your Emacs version.
 
 * Miscellaneous changes
 
+** New user option 'gnus-rcvstore-options' provides a way to
+specify additional options when saving messages to an MH folder.
+
 ** Having edited the select-method for the foreign server in the server
 buffer is immediately reflected to the subscription of the groups which
 use the server in question.  For instance, if you change

lisp/gnus/gnus-mh.el

@@ -40,6 +40,13 @@
 
 (defvar mh-lib-progs)
 
+(defcustom gnus-rcvstore-options nil
+  "Options that are passed to rcvstore, or nil.
+These are used when saving articles to an MH folder."
+  :version "26.1"
+  :group 'gnus-article
+  :type '(repeat string))
+
 (defun gnus-summary-save-article-folder (&optional arg)
   "Append the current article to an mh folder.
 If N is a positive number, save the N next articles.
@@ -77,8 +84,12 @@ Optional argument FOLDER specifies folder name."
       (save-restriction
 	(widen)
 	(unwind-protect
-	    (call-process-region
-	     (point-min) (point-max) "rcvstore" nil errbuf nil folder)
+	    (apply
+	     'call-process-region
+	     (append
+	      (list (point-min) (point-max) "rcvstore" nil errbuf nil
+		    folder)
+	      gnus-rcvstore-options))
 	  (set-buffer errbuf)
 	  (if (zerop (buffer-size))
 	      (message "Article saved in folder: %s" folder)