cangw: introduce uid command line option

cangw: introduce uid command line option

Similar to referencing iptables rules by their line number this UID allows to
reference created routing jobs, e.g. to alter configured data modifications.

The UID is an optional non-zero value which can be provided at routing job
creation time. When the UID is set the UID replaces the data modification
configuration as job identification attribute e.g. at job removal time.

The UID option is provided by mainline Linux 4.2+

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