Using port 'post' facility to proxy remove pid message to workers.

Remove pid proxying to worker engines implementation was originally
overcomplicated.  Memory pool and 2 engine posts (there and back again) are
optimized out and replaced with band new nxt_port_post() call.

src/nxt_controller.c

@@ -189,7 +189,7 @@ nxt_controller_process_new_port_handler(nxt_task_t *task,
 
     nxt_port_new_port_handler(task, msg);
 
-    if (msg->new_port->type != NXT_PROCESS_ROUTER) {
+    if (msg->u.new_port->type != NXT_PROCESS_ROUTER) {
         return;
     }