Current ICMP hard codes its forwarding rules, and never applies any
translations. Change it to use the fwd_nat_flow() function, so that it's
translated the same as TCP (excluding TCP specific port redirection).
This means that gw mapping now applies to ICMP so "ping <gw address>" will
now ping the host's loopback instead of the actual gw machine. This
removes the surprising behaviour that the target you ping might not be the
same as you connect to with TCP.
Signed-off-by: David Gibson