Process stop notification from master to all other processes.

New port message type introduced NXT_PORT_MSG_REMOVE_PID. Default handler
removes process description from nxt_runtime_t with all ports, incoming and
outgoing mmaps etc.

src/nxt_router.c

@@ -899,6 +899,7 @@ static nxt_port_handler_t  nxt_router_app_port_handlers[] = {
     nxt_port_change_log_file_handler,
     nxt_port_mmap_handler,
     nxt_router_app_data_handler,
+    nxt_port_remove_pid_handler,
 };