Refactor rt_addrmsg() and rt_routemsg().
Summary:
- Refactor rt_addrmsg(): make V_rt_add_addr_allfibs decision locally.
- Fix rt_routemsg() and multipath by accepting nexthop instead of interface pointer.
- Refactor rtsock_routemsg(): avoid accessing rtentry fields directly.
- Simplify in_addprefix() by moving prefix search to a separate function.
Reviewers: network
Subscribers: imp, ae, bz
Differential Revision: https://reviews.freebsd.org/D28011