editing: when --smooth is used, make <Enter>-at-bottom scroll one row

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.

src/text.c

@@ -1070,6 +1070,7 @@ void do_enter(void)
 #endif
 
 	refresh_needed = TRUE;
+	focusing = FALSE;
 }
 
 #ifndef NANO_TINY