When using --smooth or 'set smooth', the screen should scroll the
minimum amount needed to get the cursor back into view.  (The only
exceptions are search, undo, and redo -- when there the cursor goes
offscreen, the cursor line is centered.)

This change brings the behavior of pressing <Enter> on the bottom
row into line with, for example, pasting a single line.  See also
http://lists.gnu.org/archive/html/nano-devel/2018-02/msg00027.html.