ox-texinfo: Properly handle subauthors and email

* lisp/ox-texinfo.el (org-texinfo-template): Fix multi-line
  subauthors.  Correctly add email, when provided.  Check if author
  info is wanted first.

This patch also removes AUTHOR variable since it's up to the user to
set it or not, through a class or a header keyword.

lisp/ox-texinfo.el

@@ -682,12 +682,10 @@ holding export options."
 	 (info-filename (or (plist-get info :texinfo-filename)
 			    (file-name-nondirectory
 			     (org-export-output-file-name ".info"))))
-	 (author (org-export-data (plist-get info :author) info))
 	 (lang (org-export-data (plist-get info :language) info))
 	 (texinfo-header (plist-get info :texinfo-header))
 	 (texinfo-post-header (plist-get info :texinfo-post-header))
 	 (subtitle (plist-get info :subtitle))
-	 (subauthor (plist-get info :subauthor))
 	 (class (plist-get info :texinfo-class))
 	 (header (nth 1 (assoc class org-texinfo-classes)))
 	 (copying
@@ -715,8 +713,6 @@ holding export options."
      "\n"
      (format "@documentlanguage %s\n" lang)
      "\n\n"
-     "@c Version and Contact Info\n"
-     "@set AUTHOR " author "\n"
 
      ;; Additional Header Options set by `#+TEXINFO_HEADER
      (if texinfo-header
@@ -766,10 +762,23 @@ holding export options."
      "@title " title "\n\n"
      (if subtitle
 	 (concat "@subtitle " subtitle "\n"))
-     "@author " author "\n"
-     (if subauthor
-	 (concat subauthor "\n"))
-     "\n"
+     (when (plist-get info :with-author)
+       (concat
+	;; Primary author.
+	(let ((author (org-string-nw-p
+		       (org-export-data (plist-get info :author) info)))
+	      (email (and (plist-get info :with-email)
+			  (org-string-nw-p
+			   (org-export-data (plist-get info :email) info)))))
+	  (cond ((and author email)
+		 (format "@author %s (@email{%s})\n" author email))
+		(author (format "@author %s\n" author))
+		(email (format "@author @email{%s}\n" email))))
+	;; Other authors.
+	(let ((subauthor (plist-get info :subauthor)))
+	  (and subauthor
+	       (org-element-normalize-string
+		(replace-regexp-in-string "^" "@author " subauthor))))))
      "@c The following two commands start the copyright page.\n"
      "@page\n"
      "@vskip 0pt plus 1filll\n"