Break Netfilter and IPFW lookups out of the general either-or NAT lookup.

Break Netfilter and IPFW lookups out of the general either-or NAT lookup.

NAT lookup was prevously detemined by configure-time settings with no
possibility of two transparent options working within a single binary.

This patch moves the Netfilter and IPF lookups into sub functions and
builds a sequential failover sequence for them.

At which point any of the four transparent/interception methods may be
used simultaneoulsy on one or more listening ports.