child_process: set stdin properties when `exec`ed
When a child process is created, the stdin will not have isTTY,
isRaw and setRawMode properties. Because, uv_guess_handle in
guessHandleType call returns PIPE for fd 0. So, we create a
net.Socket and return. But normally it will return TTY and we create
tty.ReadStream and return where all those properties are properly set.
This path explicitly sets the above mentioned properties on the returned socket object.