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/proto/clipboard.pro b/src/proto/clipboard.pro
index 3dca35b..f3e8cab 100644
--- a/src/proto/clipboard.pro
+++ b/src/proto/clipboard.pro
@@ -18,7 +18,7 @@
 void clip_copy_modeless_selection(int both);
 void clip_gen_set_selection(Clipboard_T *cbd);
 int clip_gen_owner_exists(Clipboard_T *cbd);
-char *check_clipboard_option(void);
+char *did_set_clipboard(optset_T *args);
 void open_app_context(void);
 void x11_setup_atoms(Display *dpy);
 void x11_setup_selection(Widget w);