[44net] non-operational gateways

I got to thinking about Ronen's question as to how to find non-functional gateway, and I've come up with a scheme. I've added code to the rip sender that watches for ICMP unreachable packets coming back to amprgw during the rip sending cycle, which repeats every 5 minutes. If an ICMP unreachable packet is received, the matching gateway is removed from that cycle, so with luck (if the unreachables get here quickly enough) we'll only send one or two packets to a gateway that's not reachable. The time and gateway address are logged; some future process should be able to analyze this log and do something about the gateway if it's out of service long enough. (What it will do about it isn't exactly clear right now.) This doesn't seem to have slowed down the rip sender much if at all. - Brian