doc/fftools-common-opts: document -devices option

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
opts: add list device sources/sinks options

Allows to list sources/sinks of the devices that implement
that functionality.

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
avformat/rdt: Forward whitelists to rdt demuxer

Reviewed-by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>