mh-e and buffer read-only status

[Greg Hudson]

This is just a suggestion, not a bug report, so no precise version or
platform information should be needed.  I verified that emacs 21.1 has
the behavior I'm complaining about.

A user here noticed that when you use rmail, your mail messages show
up in a read-only buffer; however, when you use mh-e, the mail
messages show up in a writable buffer, although any modifications you
make to the buffer are lost (I think after prompting the user) when
you show the next message.

Perhaps mh-show-mode should set the buffer read-only.
(mh-display-message might need to be modified to temporarily turn off
read-only mode in order to put in the message content, of course.)