Re-export CGid and friends from System.Process.Internals

[RyanGlScott]

This re-exports CGid (which was previously unusable on Windows due to its unavailability) from System.Process.Internals. It also re-exports the synonyms GroupID and UserID so that the Haddocks will have a place to point to on Windows.

Fixes #90.

[RyanGlScott]

I'm not sure what went wrong with the AppVeyor build, but I don't think it was caused by these changes.

[snoyberg]

There was a recent change in AppVeyor that broke a bunch of scripts, see http://help.appveyor.com/discussions/problems/6312-curl-command-not-found. Can you rebase this PR against master? Also, can you include a comment in the changelog about the change?

[RyanGlScott]

@snoyberg, I've rebased and updated the changelog. The AppVeyor build is still failing for some reason, though.

[snoyberg]

I'm not sure what that error means either, let me play with it a bit. I'd like to hold off on merging this until AppVeyor signs off since it's obviously fairly Windows-specific.

[snoyberg]

Alright, after a ridiculous amount of pain and suffering I think I've got it fixed. Can you try rebasing against master again?

[RyanGlScott]

The Windows build is finally green! Hooray!

[snoyberg]

Woohoo!