lib: Add a function to delete all interfaces

if.c: (if_terminate) This adds a cleanup function that can be called
    when a daemon exits, similar to vty_terminate().
diff --git a/lib/if.h b/lib/if.h
index c99ab81..841ce51 100644
--- a/lib/if.h
+++ b/lib/if.h
@@ -266,6 +266,7 @@
 extern int if_is_multicast (struct interface *);
 extern void if_add_hook (int, int (*)(struct interface *));
 extern void if_init (void);
+extern void if_terminate (void);
 extern void if_dump_all (void);
 extern const char *if_flag_dump(unsigned long);