patch 9.0.0647: the 'splitscroll' option is not a good name

Problem:    The 'splitscroll' option is not a good name.
Solution:   Rename 'splitscroll' to 'splitkeep' and make it a string option,
            also supporting "topline". (Luuk van Baal, closes #11258)
diff --git a/src/structs.h b/src/structs.h
index f3e65be..d68468c 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -3621,8 +3621,8 @@
     int		w_winrow;	    // first row of window in screen
     int		w_height;	    // number of rows in window, excluding
 				    // status/command/winbar line(s)
-    int		w_prev_winrow;	    // previous winrow used for 'splitscroll'
-    int		w_prev_height;	    // previous height used for 'splitscroll'
+    int		w_prev_winrow;	    // previous winrow used for 'splitkeep'
+    int		w_prev_height;	    // previous height used for 'splitkeep'
 
     int		w_status_height;    // number of status lines (0 or 1)
     int		w_wincol;	    // Leftmost column of window in screen.