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_port.c

@@ -283,7 +283,7 @@ nxt_port_new_port_handler(nxt_task_t *task, nxt_port_recv_msg_t *msg)
 
     nxt_port_write_enable(task, port);
 
-    msg->new_port = port;
+    msg->u.new_port = port;
 }
 
 
@@ -441,6 +441,8 @@ nxt_port_remove_pid_handler(nxt_task_t *task, nxt_port_recv_msg_t *msg)
 
     nxt_memcpy(&pid, buf->mem.pos, sizeof(pid));
 
+    msg->u.removed_pid = pid;
+
     nxt_debug(task, "port remove pid %PI handler", pid);
 
     rt = task->thread->runtime;