V4L: Add internal ioctl-like interface.

This patch adds an internal ioctl-like interface which can be used in
situations where a single Video4Linux device is implemented by multiple
device drivers. One master device controls one or more slave devices.
The slaves provide Video4Linux ioctl-like interface for the use of the
master.

Only a handful of ioctls are implemented at the moment. More can (and
should) be added as more functionality is required.

Signed-off-by: Sakari Ailus <sakari.ailus@nokia.com>