canfd: add support for CAN FD in CAN_RAW sockets

- introduce a new sockopt CAN_RAW_FD_FRAMES to allow CAN FD frames
- handle CAN frames and CAN FD frames simultaneously when enabled

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>