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/option.h b/src/option.h
index 3e6f2cb..4ce009f 100644
--- a/src/option.h
+++ b/src/option.h
@@ -924,11 +924,11 @@
 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'
 EXTERN char_u	*p_swb;		// 'switchbuf'
+EXTERN char_u	*p_spk;		// 'splitkeep'
 EXTERN unsigned	swb_flags;
 // Keep in sync with p_swb_values in optionstr.c
 #define SWB_USEOPEN		0x001