[m-rev.] for review: bugfix for io.set_environment_var/4

[Julien Fischer]

On Mon, 18 Apr 2016, Mark Brown wrote:

> I agree with Julien's scheme: a portable application is likely to want
> to call these feature test predicates at the start, and decide what it
> can and can't do once and for all, rather than checking at each call
> that requires the feature. There are other possible errors though, so
> a version that returns an io.res also seems appropriate. Any other
> views on this?

Adding a version that returns an io.res is fine.  For that matter we
ought to also have versions of io.close_* that also return an error
status instead of throwing an exception.

Julien.