This is so we can reliably detect disconnect even in cases such as an
unplugged network cable: the server sends a heartbeat every 2.5 seconds. If
we fail to read anything after 5 seconds we can safely assume the server is
dead or unreachable.

See the changes to configure/CONFIG for how to enable/disable this.

Also note that enabling scan means that vcan-based version of sci is not
compiled or linked into mCANCore.

If this is not done, on 64bit linux, gcc will add padding bytes between the
header and the payload, because the header has an odd number of 32bit words.