The V4L2 plug-in is a bit peculiar with dynamically generated object
variables. We need to keep track of which controls/variables we have,
so that we can remove the variables callbacks later. The only other
way to solve this bug, that I could think of, consisted of extending
the VLC variables subsystem (which would be worse in code freeze).

This rewrite also fixes a few other bugs:
 * Support menu with non-zero based minumum choice
 * Support menu with discontinuous choices range
 * Redumdant use the extended controls API as fallback
   (This only makes sense to set more than one control at a time,
    or to set 64-bits and string controls. VLC does none of that.)
 * Unused "controls-update" and "allcontrols" variables.
 * Skipping disabled, read-only and volatile controls.

Support for the legacy control enumeration API (pre-2.6.18 kernel) is
removed; and the code is now independent of the VLC object type (it
could easily be reused for say, a V4L2 video output).