routing: Use the same index space for both nexthop and nexthop groups.

This simplifies userland object handling along with kernel-level
nexthop handling in fib algo framework.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D32342

(cherry picked from commit 7e64580b5f477ba4523799f26d1178f611f5d6cb)