bug#31737: 26.1.50; Gnus does not use mailutils movemail

[Robert Pluim]

This is based on message <87po162oze.fsf@nwalsh.com> to ding@gnus.org,
where someone noted that emacs-26 no longer installs movemail,
breaking his Gnus setup [1].

In `mail-source-movemail' in lisp/gnus/mail-source.el, Gnus only looks
in exec-directory for movemail, even though configure looked in PATH
to find mailutils movemail and decide not to install Emacs'
movemail. (rmail doesnʼt have this issue, it does runtime detection)

Gnus should probably look in at least exec-path for movemail (after
exec-directory). One way to do this would be to change the default
value of `mail-source-movemail-program to be' (executable-find
"movemail"), but that has the risk of silently switching emacs to
using mailutils' movemail instead of the emacs one.

Also, mailutils' movemail has a '--emacs' flag which rmail uses. Itʼs not
immediately obvious to me if gnus should do the same or not.

Robert

Footnotes: 
[1]  I guess people no longer use movemail that much, otherwise this
would have been noticed sooner