Add tracking for rib/nhops/nhgrp objects and provide cumulative number accessors.

The resulting KPI can be used by routing table consumers to estimate the required
scale for route table export.

• Add tracking for rib routes
• Add accessors for number of nexthops/nexthop objects
• Simplify rib_unsubscribe: store rnh we're attached to instead of requiring it up again on destruction. This helps in the cases when rnh is not linked yet/already unlinked.

Differential Revision: https://reviews.freebsd.org/D27404