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/testdir/gen_opt_test.vim b/src/testdir/gen_opt_test.vim
index 83f43f7..e0f3e48 100644
--- a/src/testdir/gen_opt_test.vim
+++ b/src/testdir/gen_opt_test.vim
@@ -134,6 +134,7 @@
       \ 'spelllang': [['', 'xxx', 'sr@latin'], ['not&lang', "that\\\rthere"]],
       \ 'spelloptions': [['', 'camel'], ['xxx']],
       \ 'spellsuggest': [['', 'best', 'double,33'], ['xxx']],
+      \ 'splitkeep': [['cursor', 'screen', 'topline'], ['xxx']],
       \ 'switchbuf': [['', 'useopen', 'split,newtab'], ['xxx']],
       \ 'tagcase': [['smart', 'match'], ['', 'xxx', 'smart,match']],
       \ 'term': [[], []],