pf: cope with IPv6 gateways for an IPv4 route in nat64

[kp]

Summary

It's possible for an IPv4 next hop to be specified as an IPv6 address. This
broke pf's route lookup in pf_route(), which is required for nat64.

Handle this case just like ip_tryforward(): use the struct sockaddr from the
struct nhop_object, and mark a struct route to indicate if_output() has to use
the gateway.

Add a test case for this.

PR: 284946
Sponsored by: Rubicon Communications, LLC ("Netgate")

[kp]

While this does fix this test case it seems to break others. I’ll debug and update.

[kp]

fixed version
(Keep rt_addrs for !nat64, use ip_dst for nat64 without gateway)

[zlei]

I have not tested this yet, the changed part of pf_route() and the new test look good to me.