bug#4431: Differences between message and mail modes

[Chong Yidong]

This bug is pretty much fixed now.  The remaining incompatibilities are
not practical to resolve, and will be caught by this change:

  2009-12-07  Chong Yidong  <cyd@stupidchicken.com>

  * simple.el (compose-mail): Check for incompatibilities and warn.
  (compose-mail-user-agent-warnings): New option.

In addition, I have finished updating the documentation in sending.texi
to deal with Message mode.

I'm closing this bug.