pfctl: delay label macro expansion until after rule optimisation

We used to expand the $nr macro in labels into the rule number prior to
the optimisation step. This would occasionally produce incorrect rule
numbers in the labels.

Delay all macro expansion until after the optimisation step to ensure
that we expand the correct values.

MFC after: 1 week
Reported by: Özkan KIRIK <ozkan.kirik@gmail.com>
Differential Revision: https://reviews.freebsd.org/D32488