patch 9.0.0445: when opening/closing window text moves up/down

Problem:    When opening/closing window text moves up/down.
Solution:   Add the 'splitscroll' option.  When off text will keep its
            position as much as possible.
diff --git a/src/optiondefs.h b/src/optiondefs.h
index 2e24cdd..8ec012f 100644
--- a/src/optiondefs.h
+++ b/src/optiondefs.h
@@ -2349,6 +2349,9 @@
     {"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},