getEnv throws an exception when the key isn't found, which is surprising and inconvenient. I'd prefer to change the signature of getEnv to return Maybe, but that will break people's code, so I'll settle for a new lookupEnv function.

It's been discussed on libraries@haskell.org and there was unanimous support.

Patches attached, the first cleans up mixed tabs and spaces (which I understand has been happening in ghc lately). The second adds the new function. Unfortunately I still can't get ghc to compile so it's untested for now. I'll keep trying with GHC, but I don't have a windows machine for testing.