v4l2: iterate controls with V4L2_CTRL_FLAG_NEXT_CTRL if possible

v4l2: iterate controls with V4L2_CTRL_FLAG_NEXT_CTRL if possible

In v2.6.18 control classes where added to the v4l2 API.
Iterating over CIDs starting with V4L2_CID_BASE will only find controls for
the first control class.
By iterating with V4L2_CTRL_FLAG_NEXT_CTRL all controls are found.

This is necessary to make controls from other control classes available in
the extra-controls property.

If V4L2_CTRL_FLAG_NEXT_CTRL is not defined at compile time or not supported
at runtime then the old mechanism for iterating is used.

https://bugzilla.gnome.org/show_bug.cgi?id=701540