[44net] amprgw and IGMP and ICMP

Recently I noticed people sending IGMP protocol packets to amprgw, which discards them (as it does for all multicast destinations sent to it). I don't think I'm going to write the necessary code to handle group membership, so dropping the packet is probably the best thing to do. Amprgw currently does NOT send 'destination unreachable' ICMP packets, including 'protocol unreachable', because those should be rate limited which makes coding the necessary response mechanism somewhat complicated. (True, I could steal the necessary code from the FreeBSD kernel and adapt it, but that's a lot of work. Maybe someday.) And I also don't want to use up encap bandwidth sending ICMP back to the AMPR hosts. The only ICMP that amprgw currently will originate is TIMXCEED when an encapped packet's TTL reaches zero, so that 'traceroute' will work. - Brian