kmscon: replace gpu selection modes with new --gpu option

kmscon: replace gpu selection modes with new --gpu option

Instead of using one option for each selection mode, we now provide a new
option --gpus. It takes as argument the selection mode to use.

We also remove the --video-devices option as it doesn't fit into the big
picture. Instead, we should provide a blacklist and users would then use
--gpu=all --blacklist-video=<blacklist> instead.

However, blacklisting is also required for input devices and until we
figure it out, we will provide no blacklisting at all.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>