V4L/DVB (4929): Read() implementation + format set/get simplifications

- implement read() entry point that works with linux list.h
- rework of VIDIOC_ENUM_FMT/VIDIOC_S_FMT/VIDIOC_G_FMT
- VIDIOC_STREAMON : allows streaming whereas there is no queued buffer
(xdtv does VIDIOC_STREAMON before VIDIOC_QBUFs)

Signed-off-by: Thierry MERLE <thierry.merle@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>