[ESS] Setting paths to R versions in different directories

[Stephen Eglen]

> One last thing, ess-r-versions doesn't sounds right to me. Shouldn't it
> be something like `ess-R-program-start-strings` or
> `ess-R-program-partial-names`?

I agree; your two choices would be more appropriate.  I doubt many
people have customized this variable, so I think renaming it shouldn't
hurt (with a defalias for the old var?).

Stephen