use separate commands for isotp PDUs

< pdu ... : is the data from the CAN ISOTP socket
< sendpdu ... : is the request from the client to send on CAN

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
only change the built-in sockopt defaults when required & cosmetics

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
doc: CAN interface configuration is currently not supported

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>