DWIM fix to message.el (TAB handling)

[Christopher Davis <ckd@loiosh.kei.com>]

The following will allow "tab" in a message-mode header to expand mail
aliases as well as completing newsgroup names.  This makes message-mode
more integrated between mail and news.

(I don't use BBDB, so if someone wants to contribute a BBDB-completion
routine for this, feel free. :-)

I think this is the Right Thing To Do with TAB in recipient headers and
should go in 5.4.16.

--- message.el~	Wed Feb 19 22:19:54 1997
+++ message.el	Tue Feb 25 15:27:07 1997
@@ -3344,13 +3344,19 @@
   "Regexp that match headers that lists groups.")
 
 (defun message-tab ()
-  "Expand group names in Newsgroups and Followup-To headers.
+  "Expand group names or mail aliases in appropriate headers.
+Will complete group names in Newsgroups and Followup-To headers,
+or mail aliases in recipient headers.
 Do a `tab-to-tab-stop' if not in those headers."
   (interactive)
-  (if (let ((mail-abbrev-mode-regexp message-newgroups-header-regexp))
+  (cond
+   ((let ((mail-abbrev-mode-regexp message-newgroups-header-regexp))
 	(mail-abbrev-in-expansion-header-p))
-      (message-expand-group)
-    (tab-to-tab-stop)))
+    (message-expand-group))
+   ((mail-abbrev-in-expansion-header-p)
+    (expand-abbrev))
+   (t
+    (tab-to-tab-stop))))
 
 (defvar gnus-active-hashtb)
 (defun message-expand-group ()