isotp[send|recv]: change command line option for extended addressing

Enable to pass the optional second extended address with the '-x' option and
do not pollute the command line options with just another '-X' option.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
isotp[send|recv]: change command line option for padding bytes

Allow to specify the TX and RX padding content separately.
Due to the optional values it is possible to provide either TX or RX values
which enables and set the given values.

-p 55:AA => TX padding 0x55, RX padding 0xAA
-p 55    => TX padding 0x55
-p 55:   => TX padding 0x55
-p :AA   => RX padding 0xAA

Additionally the comment for the -P <mode> option was updated to point out the
checking mode of the padding in the received PDUs (rx padding).

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