patch 9.1.0215: Half-page scrolling does not support smooth-scrolling

Problem:  Page-wise scrolling with Ctrl-D/Ctrl-U implements
          it's own logic to change the topline and cursor.
          More logic than necessary for scrolling with Ctrl-F/Ctrl-B
          was removed in patch 9.1.0211.
Solution: Re-use the logic from Ctrl-E/Ctrl-Y/Ctrl-F/Ctrl-B while
          staying backward compatible as much as possible.
          Restore some of the logic that determined how many lines will
          be scrolled (Luuk van Baal)

closes: #14316

Signed-off-by: Luuk van Baal <luukvbaal@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/proto/move.pro b/src/proto/move.pro
index bc9c025..91c8791 100644
--- a/src/proto/move.pro
+++ b/src/proto/move.pro
@@ -46,7 +46,6 @@
 void scroll_cursor_bot(int min_scroll, int set_topbot);
 void scroll_cursor_halfway(int atend, int prefer_above);
 void cursor_correct(void);
-int onepage(int dir, long count);
-void halfpage(int flag, linenr_T Prenum);
+int pagescroll(int dir, long count, int half);
 void do_check_cursorbind(void);
 /* vim: set ft=c : */