public void watch_child
When the process exists, the child_exited signal will be called with the
child's exit status.
Prior to calling this function, a Pty must have been set in this
using set_pty. When the child exits, the terminal's
Pty will be set to
g_child_watch_add or add_full must not have been called
child_pid, nor a SocketSource for it been created with
Note: when using the spawn_async family of functions, the
DO_NOT_REAP_CHILD flag MUST have been passed.