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

[Xavier Maillard]

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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.

Not everybody is using Ubuntu. I, for example, use Slackware and
I have no extra stuff installed than my beloved GNU Emacs ;)

	Xavier
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