Use wide Windows APIs in Cabal

[bos]

(Imported from Trac #830, reported by @batterseapower on 2011-04-11)

As part of my work on GHC's Unicode support (http://hackage.haskell.org/trac/ghc/wiki/Status/Encoding-Tickets) I identified some potential Unicode problems in Cabal install. The attached patch resolves them:

1. Use wide (UTF-16) APIs to retrieve file paths

2. Use withFilePath (from base) when marshalling a file path, as this will do the Right Thing when my changes are committed, though it is functionally equivalent to withCAString right now.

Patch attached.

[bos]

(Imported comment by @dcoutts on 2011-04-11)

Applied thanks.

Mon Apr 11 22:07:58 BST 2011  Max Bolingbroke <batterseapower@hotmail.com>
  * Unicode: use wide APIs on Windows and withFilePath on GHC