[mew-int 2954] Change in compose-mail

I am running on the developer version of emacs (yeah, I know, not
supported and so on and so forth), and I recently got bitten by this:

  * Incompatible Lisp Changes in Emacs 24.1

  ** `compose-mail' now accepts an optional 8th arg, RETURN-ACTION, and
  passes it to the mail user agent function.  This argument specifies an
  action for returning to the caller after finishing with the mail.
  This is currently used by Rmail to delete a mail window.

It would accordingly be useful if mew-user-agent-compose were changed
to accept an extra argument, even if it ignores it (for now).

- Harald