pf: make reply-to work with nat64

Just like route-to reply-to is problematic when used in combination with nat64.

In the normal (i.e. without nat64) flow we return immediately from pf_route().
However, with nat64 we need to continue and do a route lookup. In that case
we should not make the extra pf_test(PF_OUT) call to remain similar to the
non-nat64 flow.

We also have to fix the interface binding. We can only bind to the interface
after we've done the route lookup, not before.

Add a funcional test case, and a test for pfctl's rule printing.

Sponsored by: Rubicon Communications, LLC ("Netgate")