Buy some more time if the first packet to hopefully
 trigger a return set-cookie is just before the moment we'd get into fallback
 mode.
Implement ipcookie_entry_mtime_backdate_by_lifetime_log2().
Refactor dealing with ICMP packet creation into ipcookies_icmp_send().
Refactor stateless stuff into *_tateless.[ch] and implement some of the logic.