pf: Make af-to work on outbound interface

Currently af-to works only on inbound interface by creating a reversed
NAT state key which is used to match traffic returning on the outbound
interface.

Such limitation is not necessary. When an af-to state is created
for an outbound rule do not reverse the NAT state key, making it work
just like if it was created for a normal NAT rule. Depending on firewall
design it might be easier and more natural to use af-to on the outbound
interface.

Reviewed by: kp
Approved by: kp (mentor)
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D49122