This is the main control Stdio.Fd and is always on fd number 3.
3
To spawn a new process, a new Stdio.PROP_SEND_FD capable Stdio.Fd is sent over this fd, and a single byte of data is sent as payload.
The sent fd will become a ForkFd inside a ForkStream .
inherit Stdio.Fd : Fd
Tools.Standalone.forkd