bug#26102: movemail can't connect mail server

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Fri, 17 Mar 2017 11:49:13 +0100
> Cc: eggert@cs.ucla.edu, hengaini2055@qq.com
> 
> Paul Eggert <eggert@cs.ucla.edu> writes:
> 
> > As the default Emacs configuration still has a significant security
> > hole, this patch changes 'configure' to warn about the problem if
> > present.
> 
> Is there any reason rmail doesn't just use pop3.el instead of the
> movemail binary when speaking to pop3 servers?

I guess because no one wrote the code to support that.  movemail
supports many different protocols, including /var/mail files etc., so
Rmail needs just rudimentary understanding of the inbox syntax spec to
invoke movemail correctly.  To single out POP3 would need a bit more
code, but nothing too complicated, of course.

Patches welcome.  (For backward compatibility, the old code that uses
movemail for POP3 should still be kept, I think, subject to some user
option.)