Rework part of routing code to reduce difference to D26449.

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