slcand: add slcan_attach and UART settings

slcand: add slcan_attach and UART settings

slcand takes settings known from slcan_attach to configure CAN
interface, like open/close channel, set bitrate etc (Oliver Hartkopp).

In addition -S option sets UART's baudrate (Yegor Yefremov).

This way slcand combines three autilities in one: old slcand,
slcan_attach and stty.

Example: slcand -o -s8 -S 3000000 ttyUSB0

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Reviewed-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>