network/nexthop: manage all nexthops by manager

network/nexthop: manage all nexthops by manager

The kernel manages nexthops by their IDs. Previously networkd manages
nexthops in three ways:
- by the corresponding link, if a nexthop has ifindex,
- by the manager, if a nexthop does not have ifindex,
- by the manager with their IDs.

This unifies the three managements of nexthops into one, and use the
same way as the kernel uses.

This is the one for nexthop already done by
aa9626ee3b96e0d2a9a816b5efd38fd7dc829def for neighbor.