V4L/DVB: V4L: Events: Add new ioctls for events

This patch adds a set of new ioctls to the V4L2 API. The ioctls conform to
V4L2 Events RFC version 2.3:

<URL:http://www.spinics.net/lists/linux-media/msg12033.html

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Signed-off-by: Sakari Ailus <sakari.ailus@maxwell.research.nokia.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>