Handle EHOSTDOWN and refine error handling better granularity

[arekm]

Handle EHOSTDOWN which occurs on MacOS and causes:

% mtr 192.168.68.2
mtr: Unexpected mtr-packet error

for unreachable host and also split error handling better granularity.

[arekm]

That's continuation of #504

[arekm]

There is one thing there:

} else if (icmp_type == ICMP_DEST_UNREACH) {
-        result = "no-route";
+        /* XXX icmphdr->code is not known here, so assume that host is unreachable */
+        result = "no-route-host";

accessing icmphdr->code for ipv4 is possible but requires a bit of changes and not sure if it's worth the hassle.

Also ipv6 has no host vs network unreachable differentiation as I see it:
https://www.iana.org/assignments/icmpv6-parameters/icmpv6-parameters.xhtml#icmpv6-parameters-3