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

[Paul Eggert]

I just proposed a simple patch for the movemail $PATH problem here:

https://lists.gnu.org/r/emacs-devel/2018-06/msg00441.html

But now I see that Robert proposed a slightly-more-complicated patch here:

https://debbugs.gnu.org/31737#63

I think either will work, and that one of the two should be applied to 
the emacs-26 and master branches.