Implement kernel support for RFC 5549/8950.
* Relax control plane restrictions and allow specifying IPv6 gateways
for IPv4 routes. This behavior is controlled by the
net.route.rib_route_ipv6_nexthop sysctl (on by default).
* Always pass final destination in ro->ro_dst in ip_forward().
* Use ro->ro_dst to exract packet family inside if_output() routines.
Consistently use RO_GET_FAMILY() macro to handle ro=NULL case.
* Pass extracted family to nd6_resolve() to get the LLE with proper encap.
It leverages recent lltable changes committed in c541bd368f.
Presence of the functionality can be checked using ipv4_rfc5549_support feature(3).
Example usage:
route add -net 192.0.0.0/24 -inet6 fe80::5054:ff:fe14:e319%vtnet0
Differential Revision: https://reviews.freebsd.org/D30398
MFC after: 2 weeks
62e1a437f3