pf: Make af-to less magical

I didn't know BSD had an IPv4/IPv6 translation mechanism built-in. On Linux the state of the art seems to be Jool[1], which is unfortunately an out-of-tree kernel module. IIUC, they currently share the limitation of not being able to translate locally-originated packets, which can be annoying unless you have a machine to dedicate to the translation.

[1]: https://nicmx.github.io/Jool/en/intro-jool.html