[PATCH] IPv4 over IPv6 nexthop support on FreeBSD

Hello, While experimenting in FreeBSD 13.1 with IPv4 over IPv6 nexthop routes, I realized that BIRD was unable to read routes from the kernel as well as export routes to the kernel. The support for it was implemented in FreeBSD 13.1 [1]. I have attached a patch that made it work for me on FreeBSD 13.1, both importing and exporting to/from kernel. I believe the patch will still need some work, as from a quick look, OpenBSD still has no support for it. I'm happy to implement any suggested improvements and send a v2 of it. Thank you, Luiz [1] https://reviews.freebsd.org/D30398