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.

PR: 252744
MFC: 1 week
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