Set the exec() status of the child

Set the exec() status of the child before reading from the child's
stdin/stderr: the exec status determines the message type (proxy or
stdout) returned by the port.

Begin recording the exit status of the child. Currently it is just a
non-zero number. It should be set to the actual exit status of the
child. The value should be reported to the erlang side as well:

    {exit_status, integer()}