V4L/DVB (4190): Add helper functions for control processing to v4l2-c…

…ommon.

Control processing is often duplicated in the various drivers. Unfortunately,
simple things like the names of controls are often different between drivers,
even though it is the same controls. Adding in the new extended controls and
the need for having control helper functions became apparent.
Several functions have now been added to v4l2-common to do things like
filling the v4l2_queryctrl and v4l2_querymenu structs, to check for
valid control input and to move to the next control when enumerating
over all controls.

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