patch 9.0.1330: handling new value of an option has a long "else if" chain

Problem:    Handling new value of an option has a long "else if" chain.
Solution:   Use a function pointer. (Yegappan Lakshmanan, closes #12015)
diff --git a/src/clipboard.c b/src/clipboard.c
index e140113..b09c678 100644
--- a/src/clipboard.c
+++ b/src/clipboard.c
@@ -1253,7 +1253,7 @@
  * Return an error message or NULL for success.
  */
     char *
-check_clipboard_option(void)
+did_set_clipboard(optset_T *args UNUSED)
 {
     int		new_unnamed = 0;
     int		new_autoselect_star = FALSE;