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/option.h b/src/option.h
index ab34e82..97b7a6c 100644
--- a/src/option.h
+++ b/src/option.h
@@ -924,6 +924,7 @@
 EXTERN char_u	*p_sps;		// 'spellsuggest'
 #endif
 EXTERN int	p_spr;		// 'splitright'
+EXTERN int	p_spsc;		// 'splitscroll'
 EXTERN int	p_sol;		// 'startofline'
 EXTERN char_u	*p_su;		// 'suffixes'
 EXTERN char_u	*p_sws;		// 'swapsync'