revert last patch, and do it differently: set /input/joysticks/which (int)

revert last patch, and do it differently: set /input/joysticks/which (int)
to the joystick index (x in /input/joysticks/js[x]) both when initializing
nasal init blocks & whenever updating the joystick. That's the only way to
make these nasal contexts aware of the joytick for which they were called.
And that's useful if a js maintains common variables, such as modifiers or
state variables, that should be shared between the namespace-less bindings
and initialization code. And, yes, it's over-engineered ...  :-)