Update syntax of Haddock module attributes

This re-hides several modules that had been inadvertently unhidden by
the change in attribute syntax in Haddock 2.0.0.0 (nearly five years
ago!)
Merge pull request #1153 from benmachine/haddockattributes

Haddock module attributes specified with old syntax
Rename D.C.SetEnv to D.C.Env.
Add lookupEnv to D.C.Env.
Don't die when the PATH variable is missing.