SocketCAN DUNE implementation

CANbus support was not present in DUNE, and therefore had to be added. Many DUNE tasks use an interface defined by IO::Handle, and so does this SocketCAN implementation. Apart from the setup, using CAN thus becomes identical to using Serial or TCP in DUNE. This also enables use of the Poll::poll function, and makes it possible to only read the SocketCAN buffer when there is something in it. Reading it without checking will result in the task waiting until a CAN message is received. An example of reading with