Align with the new CAN frame structure for socketcan adding optional DLC element

[rahagal]

Is your feature request related to a problem? Please describe.

SocketCAN allows for passing an optional DLC element in its can_frame structure in addition to the length element, this allow accessing the raw DLC value that can be 0-15 (with 9-15 interpreted as 8 in classic CAN).
This feature is good for testing compliance with the ISO.
To enable this some changes are implemented in the Linux kernel and in can-utils.
You can see the change being introduced to can-utils in here: linux-can/can-utils@c398e56

Describe the solution you'd like

I would like to suggest an adaptation to the way CAN_FRAME_HEADER_STRUCT is being defined and un/packed.

Describe alternatives you've considered

I did not find a way to achieve this with the current implementation.

Additional context

See a patch used in a fork I've created that allowed me to achieve this capability.
socktcan_over8_dlc.patch