[ESS] Setting paths to R versions in different directories

[Stephen Eglen]

> How about having only `R-program-start-regexp` and set it to "^R-"?

okay, but "start-regexp" is confusing:  1) if it
says start, would you expect to have to add ^ [caret] for start of
string 2) ess-find-exec-completions() does the hard work of finding the
R versions, and a quick browse suggests that ess-r-versions should
contain strings, not regexps.

S