pf: g/c unneeded af (address family) params to pf_change_ap

both af and naf (af-to case) are in the pf_pdesc
some code shuffling to actually set these before calling pf_change_ap
inspired by Richard Procter <richard.n.procter@gmail.com>'s mail on tech
from Aug 17, but redone
ok bluhm vgross

Obtained from: OpenBSD, henning <henning@openbsd.org>, 78ad05cbd1
Sponsored by: Rubicon Communications, LLC ("Netgate")