Fix start-up order for making DHCPv6 DUID

Previously, if the DUID wasn't read from the lease-file or
script, a new one was created _after_ the helper process fork,
so for that first run, the script calls got an empty DUID.

Also, use a DUID_LL format DUID when there's no stable lease
storage, as well as when the RTC is broken. That has a chance of
evaluating to the same value on each startup.

src/dnsmasq.h

@@ -1120,6 +1120,7 @@ u64 lease_find_max_addr6(struct dhcp_context *context);
 void lease_ping_reply(struct in6_addr *sender, unsigned char *packet, char *interface);
 void lease_update_slaac(time_t now);
 void lease_set_iaid(struct dhcp_lease *lease, int iaid);
+void lease_make_duid(time_t now);
 #endif
 void lease_set_hwaddr(struct dhcp_lease *lease, unsigned char *hwaddr,
 		      unsigned char *clid, int hw_len, int hw_type, int clid_len, time_t now, int force);