v2020.12.0

candump: Enable zero relative timestamps to be used in log files

The logfile format which is generated with '-l' and '-L' consists of an
absolute epoch timestamp. In some use cases (e.g. for documentation) a
zero relative timestamp can be useful which was only configurable with
the '-tz' option for the classic output.

'-tz' and '-ta' are now valid options for the logfile format.

Signed-off-by: Richard Young <code@richyoung.ca>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>