Generate correct ICMP for unreachable nodes

[iskra]

In IP forwarding code, either over tree or over torus, where the destination node is unreachable, we should ideally generate and ICMP "host unreachable" packet and send it back to the sender. I did that for the tree network, but tests with "ping" indicate that the return packet is not properly recognized by the kernel, even though tests with tcpdump did not reveal any obvious corruption in the packet.