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/runtime/optwin.vim b/runtime/optwin.vim
index 04fe3b3..018dbfc 100644
--- a/runtime/optwin.vim
+++ b/runtime/optwin.vim
@@ -516,10 +516,10 @@
 call <SID>OptionG("swb", &swb)
 call <SID>AddOption("splitbelow", gettext("a new window is put below the current one"))
 call <SID>BinOptionG("sb", &sb)
+call <SID>AddOption("splitkeep", gettext("determines scroll behavior for split windows"))
+call <SID>BinOptionG("spk", &spk)
 call <SID>AddOption("splitright", gettext("a new window is put right of the current one"))
 call <SID>BinOptionG("spr", &spr)
-call <SID>AddOption("splitscroll", gettext("determines scroll behavior for split windows"))
-call <SID>BinOptionG("spsc", &spsc)
 call <SID>AddOption("scrollbind", gettext("this window scrolls together with other bound windows"))
 call append("$", "\t" .. s:local_to_window)
 call <SID>BinOptionL("scb")