Tweak strategy for confirming SLAAC addresses.

The code which conirms possible SLAAC addresses associated with
hosts known from DHCPv4 addresses keeps trying at longer and longer
intervals essentially forever, EXCEPT if sending an ICMP ping results
in a HOSTUNREACH error, which terminates the process immediately.

It turns out that this is too drastic. Routing changes associated
with addressing changes can cause temporary HOSTUNREACH problems,
even when an address has not gone forever. Therefore continue
trying in the face of HOSTUNREACH for the first part of the
process. HOSTUNREACH errors will still terminate the process
after it reaches the slow tail of retries.

Thanks to Andrey Vakhitov for help diagnosing this.

src/slaac.c

@@ -166,7 +166,8 @@ time_t periodic_slaac(time_t now, struct dhcp_lease *leases)
 	    
 	    if (sendto(daemon->icmp6fd, daemon->outpacket.iov_base, save_counter(-1), 0,
 		       (struct sockaddr *)&addr,  sizeof(addr)) == -1 &&
-		errno == EHOSTUNREACH)
+		errno == EHOSTUNREACH &&
+		slaac->backoff == 12)
 	      slaac->ping_time = 0; /* Give up */ 
 	    else
 	      {