Update nexthop handling for route addition/deletion in preparation for mpath.

Currently kernel requests deletion for the certain routes with specified gateway,
but this gateway is not actually checked. With multipath routes, internal gateway
checking becomes mandatory. Add the logic performing this check.

Generalise RTF_PINNED routes to the generic route priorities, simplifying the logic.

Add lookup_prefix() function to perform exact match search based on data in @info.

Differential Revision: https://reviews.freebsd.org/D26356