lib: add support for non-zero dlc values in CAN 2.0B RTR frames

lib: add support for non-zero dlc values in CAN 2.0B RTR frames

As Hakan Engblom pointed out a CAN 2.0B RTR frame can contain a non-zero DLC
value which was not addressed by parse_canframe() and sprint_canframe().

This patch adds support for non-zero DLC values in a way that legacy logfiles
are still usable as the DLC can optionally be added.

Reported-by: Hakan Engblom <H.Engblom@tele-radio.com>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>