Preserving message 'share' field when pushing to queue.

As far as I understand, this field is important to control the number of
buffers send in a single write attempt.  Furthermore, having uninitialized
field is always bad.

This closes #204 issue on GitHub.
Thanks to 洪志道 (Hong Zhi Dao).

src/nxt_port_socket.c

@@ -179,6 +179,7 @@ nxt_port_msg_create(nxt_task_t *task, nxt_port_send_msg_t *m)
     msg->link.prev = NULL;
 
     msg->buf = m->buf;
+    msg->share = m->share;
     msg->fd = m->fd;
     msg->close_fd = m->close_fd;
     msg->port_msg = m->port_msg;