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/globals.h b/src/globals.h
index aa0efb8..ebd9c59 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1975,10 +1975,10 @@
 EXTERN optmagic_T magic_overruled INIT(= OPTION_MAGIC_NOT_SET);
 
 #ifdef FEAT_CMDWIN
-// Skip win_fix_cursor() call for 'nosplitscroll' when cmdwin is closed.
+// Skip win_fix_cursor() call for 'splitkeep' when cmdwin is closed.
 EXTERN int skip_win_fix_cursor INIT(= FALSE);
 #endif
-// Skip win_fix_scroll() call for 'nosplitscroll' when closing tab page.
+// Skip win_fix_scroll() call for 'splitkeep' when closing tab page.
 EXTERN int skip_win_fix_scroll INIT(= FALSE);
 // Skip update_topline() call while executing win_fix_scroll().
 EXTERN int skip_update_topline INIT(= FALSE);