Port message fragmentation supported.

- Each sendmsg() transmits no more than port->max_size payload data.
- Longer buffers are fragmented and send using multiple sendmsg() calls.
- On receive side, buffers are connected in chain.
- Number of handler calls is the same as number of nxt_port_socket_write()
  calls.
- nxt_buf_make_plain() function introduced to make single plain buffer from
  the chain.

src/nxt_port.c

@@ -248,6 +248,8 @@ nxt_port_new_port_handler(nxt_task_t *task, nxt_port_recv_msg_t *msg)
 
     new_port_msg = (nxt_port_msg_new_port_t *) msg->buf->mem.pos;
 
+    /* TODO check b size and make plain */
+
     nxt_debug(task, "new port %d received for process %PI:%d",
               msg->fd, new_port_msg->pid, new_port_msg->id);