Add IPv4/IPv6 rtentry prefix accessors.

Multiple consumers like ipfw, netflow or new route lookup algorithms
need to get the prefix data out of struct rtentry.
Instead of providing direct access to the rtentry, create IPv4/IPv6
accessors to abstract struct rtentry internals and avoid including
internal routing headers for external consumers.

While here, move struct route_nhop_data to the public header, so external
customers can actually use lookup functions returning rt&nhop data.

Differential Revision: https://reviews.freebsd.org/D27416