CSMA bugfix

This bugfix fixes a pretty severe problem that may heavily affect fragmented IPv6 packets in case of packet loss. The problem was that the CSMA code blindly would remove the first packet on the output queue when the CSMA callback was received, even if the callback pertained to another packet. To make matters worse, the code would update the packet attributes of the transmitted packet so that the sequence number might become off by one.

This bugfix ensures that the right packet on the queue is removed.