window port interface change - add mbehavior flags to start_menu()

Provide a way to communicate additional behaviors and/or appearances
desired from NetHack window port menus.

This is foundation work for changes to follow at a future date.

include/wintype.h

@@ -111,6 +111,16 @@ typedef struct mi {
 #define MENU_ITEMFLAGS_SELECTED   0x0000001U
 #define MENU_ITEMFLAGS_SKIPINVERT 0x0000002U
 
+/* 3.7+ enhanced menu flags that not all window ports are likely to
+ * support initially.
+ *
+ * As behavior and appearance modification flags are added, the various
+ * individual window ports will likely have to be updated to respond
+ * to the flags in an appropriate way.
+ */
+
+#define MENU_BEHAVE_STANDARD      0x0000000U
+
 /* clang-format on */
 
 #endif /* WINTYPE_H */