V4L/DVB (5271): Add VIDIOC_TRY_ENCODER_CMD and VIDIOC_ENCODER_CMD ioctls.

Add support for starting, stopping, pausing and resuming an MPEG (or similar
compressed stream) encoder.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>