slcand: improve usage examples

The examples in the usage of slcand use a 'ttyslcan0' device which
is an uncommon name.  Use a more common ttyUSB0 name which is seen
with CANUSB devices.  Also, add an example showing that /dev/ttyUSB0
can be use as well.

Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
slcand: remove #define EXIT_FAILURE, EXIT_SUCCESS

slcand has #defines for EXIT_FAILURE and EXIT_SUCCESS but this is
unnecessary since they are already provided by stdlib.h.  Remove them.

Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
slcand: consistent use of exit(EXIT_FAILURE)

Some places used exit(1) and others used exit(EXIT_FAILURE).
Be consistent and use exit(EXIT_FAILURE) for all of these instances.

Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
use line discipline number from linux/tty.h

The line discipline number is provided in include/linux/tty.h.
Use that instead of a #define in each program.

Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>