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

[Aaron Bentley]

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Vincent Ladeuil wrote:
>     aaron> Though I still have doubts about whether we should be
>     aaron> supporting two different ways of sending mail via
>     aaron> emacs...
> 
> Look at it that way:
> 
> We should support several different ways of sending mail from
> Ubuntu because there exists several available MUAs.

Note that Ubuntu *does* provide a standard way of launching the user's
preferred mail client: xdg-email.  Windows and Mac also have ways of
launching the preferred mail client.

> Now replace Ubuntu by emacs.

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

> And I'm serious :)

I've seen lots of people claim that Emacs is a operating system, not an
editor.  But this is the first time I've seen an Emacs user admit it.

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

- - xdg-utils isn't installed everywhere (62% of Ubuntu machines have it,
  according to popcon.ubuntu.com)
- - thunderbird is quirky, and supporting it directly seems to work best.

Aaron
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