v4l2: v4l2_camera_file: Add V4L2CameraFile to model the opened camera file

With relation to opening files, the kernel has three objects related to
files:
- inodes, that represent files on disk
- file objects, that are allocated at open() time and store all data
  related to the open file
- file descriptors, that are integers that map to a file

In the V4L2 compatibility layer, V4L2CameraProxy, which wraps a single
libcamera camera via V4L2Camera, is more or less equivalent to the
inode. We also already have file descriptors (that are really eventfds)
that mirror the file descriptors. Here we create a V4L2CameraFile to
model the file objects, to contain information related to the open file,
namely if the file has been opened as non-blocking, and the V4L2
priority (to support VIDIOC_G/S_PRIORITY later on). This new class
allows us to more cleanly support multiple open later on, since we can
move out of V4L2CameraProxy the handling of mapping the fd to the open
file information.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

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