Complete rework of CAN netlink gateway.

Major changes:

- rework internal structures to prepare routings and modifications of CAN traffic also to non-CAN interfaces
- fix reading of netlink messages in cangw.c (added RTCAN_RTA / RTCAN_PAYLOAD macros)
- rework reading of gw-job lists in cgw_dump_jobs()
- rename of functions and API definitions to have a common namespace cgw_
- added infrastructure to perform crc8 and xor checksums in CAN frame data[]

TODO:

- add and test functionality for crc8 and xor checksums in CAN frame data[]
- add help text for crc8 and xor checksums in CAN frame data[]