Refactor rt_addrmsg() and rt_routemsg().

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