Make eww rendering default to `window-width' again

* lisp/gnus/mm-decode.el (mm-shr): Bind `shr-width' to `fill-column'
so that lines don't get overlong when responding.

* lisp/net/eww.el (eww-mode): Truncate overlong lines for prettier
display when resizing.

* lisp/net/shr.el (shr-width): Default to using the window width when
rendering.
lisp/gnus/gnus-cloud.el: Start collecting a newsrc