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

[melifaro]

Summary

Currently kernel requests deletion for the certain routes with gateway, but this gateways is not actually checked.
With multipath, checking that the code deleting the right route is necessary. 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.