Support rdr pass

[issackelly]

After spending much quality time with various pfctl implementations, I think natpass needs to be set in order to support rdr pass type rules. Patch to our vendored folder is attached.

Sorry for the old-school patch file. We're currently vendored to make some interfaces public and because we would like to add transaction rollback support as well, which is in progress and we would be happy to upstream once it's finished if you're interested.

rdr-pass.diff.txt