x/sys/unix: add CAN error constants from linux/can/error.h

[skladd]

Currently these error constants like CAN_ERR_BUSOFF are not included in zerrors_linux.go.
Adding #include <linux/can/error.h> to mkerrors.sh would add 58 constants allowing interpretation of SocketCAN error frames.

[gopherbot]

Change https://golang.org/cl/251037 mentions this issue: unix: add CAN error constants

[cagedmantis]

/cc @ianlancetaylor @bradfitz @tklauser