Rework part of routing code to reduce difference to D26449.

• Split rt_setmetrics into get_info_weight() and rt_set_expire_info(), as these two can be applied at different entities and at different times.
• Start filling route weight in route change notifications
• Pass flowid to UDP/raw IP route lookups
• Rework nd6_subscription_cb() and sysctl_dumpentry() to prepare for the fact that rtentry can contain multiple nexthops.

Differential Revision: https://reviews.freebsd.org/D26497