On the one hand, at this point, none of the variables that make use of
a choices list need it enforced. On the other hand, some variables
really provide a non-exhaustive list of suggestions.

Specifically, the following variables provide mere suggestions:
visual, intf-add, intf-skins, aspect-ratio, crop and zoom.

The following variables check against invalid values to fail safe:
program, audio-es, video-es, nav-es, spu-es, bookmark, title, chapter,
deinterlace, deinterlace-mode, the title N variables and the V4L2
control variables.

The postproc-q variable enforces sane values via mininum and maximum.

Finally, the following variables provide a purely informational list
and ignore the variable value: teletext-es and controls (V4L2).