Watches child_pid.
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 null.
Note: g_child_watch_add or add_full must not have been called for child_pid, nor a
SocketSource for it been created with ChildWatchSource.
Note: when using the spawn_async family of functions, the DO_NOT_REAP_CHILD flag MUST have been passed.
| this |
a Terminal |
| child_pid |
a Pid |