Adds ability to start Funnel in the foreground and stream incoming
connections. When foreground process is stopped, Funnel is turned
back off for the port.
Exampe usage:
```
TAILSCALE_FUNNEL_V2=on tailscale funnel 8080
```
Updates #8489
Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>