Add a control socket server and CLI client for runtime management
of Gunicorn instances, similar to birdc for BIRD routing daemon.
Features:
- Control socket server running in arbiter process (asyncio/threaded)
- gunicornc CLI with interactive and single-command modes
- JSON protocol with length-prefixed framing
- Commands: show workers/stats/config/listeners/dirty, worker add/remove/kill,
dirty add/remove, reload, reopen, shutdown
- Stats tracking (uptime, workers spawned/killed, reloads)
- Configurable socket path and permissions
New config options:
- control_socket: Unix socket path (default: gunicorn.ctl)
- control_socket_mode: Socket permissions (default: 0o600)
- --no-control-socket: Disable control socket
