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