Notify embedded terminal on embedded process exit

When an embedded process exits the main process receives a SIGCHLD. The
check on whether the embedded process is alive is done on input, so
forward an input of '\0' to get the embedded terminal to notice its
child is dead.

src/bin.rs

@@ -528,6 +528,11 @@ fn run_app() -> Result<()> {
                                 state.redraw();
                             }
                         },
+                        signal_hook::SIGCHLD => {
+                            state.rcv_event(UIEvent::EmbedInput((Key::Null, vec![0])));
+                            state.redraw();
+
+                        }
                         other => {
                             debug!("got other signal: {:?}", other);
                         }