Added automatic extended frame format recognition.

Added automatic extended frame format recognition.

Most SocketCAN userspace tools (like cansend) recognize the EFF by the
stringlength of the given CAN-ID: 3 -> SFF , 8 -> EFF.
This was missing in the candump filter definition and therefore the
values had to be specified with the CAN_EFF_FLAG set in the value, e.g.
92345678 for the extended CAN-ID 12345678 - this was not really nice.

Now the filtersets for extended frames can be specified as expected:
12345678:1FFFFFFF to filter for only 12345678 extended CAN-ID
000000AA:1FFFFFFF to filter for only AA extended CAN-ID.