sbin/ipfw: Fix parsing error in table based forward

The argument parser does not recognise the optional port for an
"tablearg" argument. Fix simplifies the code by make the internal
representation expicit for the parser. Includes the fix from D30208.

PR: 252744
Reported by: <bugs.freebsd.org@mx.zzux.com>
Approved by: nc
Tested by: <bugs.freebsd.org@mx.zzux.com>
Differential Revision: https://reviews.freebsd.org/D30164

(cherry picked from commit 6cb13813caa09305046e0cecad8bba3ae2287b0d)
(cherry picked from commit f6f297871d469daf808f78faead8f950a2c81e36)