[MERGE] Add mail-mode GNU Emacs mail package as a mail_client option.

[Xavier Maillard]

       aaron> If Emacs is an OS, why doesn't it have such a facility?

   Good point. 

It has:

mail-user-agent is a variable defined in `simple.el'.
Its value is sendmail-user-agent

Documentation:
*Your preference for a mail composition package.
Various Emacs Lisp packages (e.g. Reporter) require you to compose an
outgoing email message.  This variable lets you specify which
mail-sending package you prefer.

Valid values include:

  `sendmail-user-agent' -- use the default Emacs Mail package.
                           See Info node `(emacs)Sending Mail'.
  `mh-e-user-agent'     -- use the Emacs interface to the MH mail system.
                           See Info node `(mh-e)'.
  `message-user-agent'  -- use the Gnus Message package.
                           See Info node `(message)'.
  `gnus-user-agent'     -- like `message-user-agent', but with Gnus
                           paraphernalia, particularly the Gcc: header for
                           archiving.

Additional valid symbols may be available; check with the author of
your package for details.  The function should return non-nil if it
succeeds.


   But then Xaviers's patch may have to be tweaked: instead of
   trying to integrate one set of emacs constraints into bzr,
   integrate just a minimal call through xdg-email (or not) and
   handle all the emacs related customizations* in emacs itself**.

I do not see why we would want this especially when xdg-* is not
present on the user machine.

       aaron> Bazaar shouldn't need its own facility.  It has one because:

       aaron> - xdg-utils isn't installed everywhere (62% of Ubuntu
       aaron>   machines have it, according to popcon.ubuntu.com)

   Then having a generic emacs ExternalMailClient may be appropriate
   (xdg-utils may not be available on OSX nor windows nor Solaris,
   etc).

I second this.

       aaron> - thunderbird is quirky, and supporting it directly
       aaron> seems to work best.

   Thanks for implying that emacs is not quirky :-P

   More seriously, I can understand the concerns about supporting
   every possible MUA in bzr core, but why not providing plugins
   then ? We should find the right balance between 'batteries
   included' and 'everything and the kitchen sink included'.

   Finally another API may be around mailto URLs...

Could you elaborate on this ?

	  Vincent

   *: emacs version (why require 22.1 ?), rmail or gnus, specific
    ways to add attachment (including inline), required in-buffer
    movements, etc.

because emacsclient -e (or --eval) did not exist for emacs prior
to version 22.1.

	Xavier
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