bug#5299: Default mailer (was: custom-reevaluate-setting / custom-initialize-delay)

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> Thanks, so I think the patch below is needed, can you try it to see if
>> it helps?
> Nope.
> I loaded a patched version of gnus-art.el, then tried to send a bug report.
> I got only this (in *Messages*):

> Sending...
> Sending via mail...
> message-send-mail-with-sendmail: Sending...failed with exit value 1

I guess that good: it means that my patch worked and then you bumped
into the next problem which is that message-send-mail-function is not
set to use mailclient.

I think we need to revisit the setting for the default mailer: I think
neither the mailclient-send-it nor any other setting is a good default.
I think we should instead use as default a special function that prompts
the user whether he wants to use his external MUA to send email
(i.e. use mailclient-send-it), or to use /usr/sbin/sendmail, or to use
smtpmail in which case it'll ask the user to configure his SMTP server.

And this should be used on *all* systems rather than only under w32
and/or macosx, since sadly /usr/sbin/sendmail is too often unusable even
under GNU/Linux (my /usr/sbin/sendmail is usable, but I must admit that
it took me a significant amount of work on my laptop, so I can't expect
most people to go through that trouble).


        Stefan