When cgroup-network calls fatal() after creating its spawn server,
the spawn server process is orphaned and reparented to PID 1 (netdata
in Docker). When the orphaned spawn server exits, it becomes a zombie
because netdata is not reaping non-child processes.

This fix registers a fatal callback that properly destroys the spawn
server before exit, preventing orphaning.

Changes:
- Add cleanup_spawn_server_on_fatal() callback
- Register callback immediately after spawn server creation
- Set spawn_server = NULL after destruction in both cleanup paths

Fixes: netdata#20565