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/optiondefs.h b/src/optiondefs.h
index de04abe..2ce3ec3 100644
--- a/src/optiondefs.h
+++ b/src/optiondefs.h
@@ -2350,12 +2350,12 @@
     {"splitbelow",  "sb",   P_BOOL|P_VI_DEF,
 			    (char_u *)&p_sb, PV_NONE,
 			    {(char_u *)FALSE, (char_u *)0L} SCTX_INIT},
+    {"splitkeep",   "spk",  P_STRING,
+			    (char_u *)&p_spk, PV_NONE,
+			    {(char_u *)"cursor", (char_u *)"cursor"} SCTX_INIT},
     {"splitright",  "spr",  P_BOOL|P_VI_DEF,
 			    (char_u *)&p_spr, PV_NONE,
 			    {(char_u *)FALSE, (char_u *)0L} SCTX_INIT},
-    {"splitscroll", "spsc", P_BOOL,
-			    (char_u *)&p_spsc, PV_NONE,
-			    {(char_u *)TRUE, (char_u *)TRUE} SCTX_INIT},
     {"startofline", "sol",  P_BOOL|P_VI_DEF|P_VIM,
 			    (char_u *)&p_sol, PV_NONE,
 			    {(char_u *)TRUE, (char_u *)0L} SCTX_INIT},