Slightly-breaking performance improvement

Modified

• The message is not copied into the node anymore, it is removed from the
  vcan_node_t struct. Instead it is copied once and only once into the
  vcan_bus_t struct, to avoid copying it into every single node.
• Due to the upper point, the callbacks now obtain also a const pointer to the
  message as a second parameter.

Fixed

• Minor doxygen documentation improvements.