V4L/DVB (13908): cx18: Add initial working VIDIOC_G_ENC_INDEX ioctl() support

VIDIOC_G_ENC_INDEX support see the light of day.
Some notes:
1. With default capture parameters, the CX23418 seems to transfer 192 index
entries (4.5 kB worth) at 10 second intervals.

2. Index streams don't seem to be supported for MPEG 2 TS streams

3. The index entries seem to claim every frame is a B-Frame.  Possible
firmware bug.

4. The cx18 driver does not try to capture an index stream when inserting
sliced VBI into the MPEg stream as the offsets would need fixup.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>