If packet has been NAT'd then we certainly need to recalculate the checksum,
but even for direct pass-through it might have been received with an invalid
checksum due to checksum offload. For now, recalculate full checksum in all
cases.
See #1.
Added explicit NAT target, allowing NAT even within client net and
making it clear that NAT is used externally.
Changed Redirect_to_netvm to NAT_to, and allow specifying any target
host.