Changed read_msg_fd() to always use send_msg()/rwrite() instead of

Changed read_msg_fd() to always use send_msg()/rwrite() instead of
sometimes calling io_multiplex_write()/rwrite() or msg_list_add().
This simplifies the code that does the defer_forwarding_messages
checking because it can now be done in just one place, in send_msg()
(because rwrite() also calls send_msg() now when forwarding a msg).