routing: Allow using IPv6 next-hops for IPv4 routes (RFC 5549).

routing: Allow using IPv6 next-hops for IPv4 routes (RFC 5549).

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 c541bd368f86.

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