Fix an egregious strictness analyser bug (Trac #4924)

The "virgin" flag was being threaded rather than treated
like an environment.  As a result, the second and subsequent
recursive definitions in a module were not getting a
correctly-initialised fixpoint loop, causing much worse
strictness analysis results.  Indeed the symptoms in
Trac #4924 were quite bizarre.

Anyway, it's easily fixed.  Merge to stable branch.