Based on http://hackage.haskell.org/trac/ghc/ticket/3307 ,
whether FilePath contains decoded unicode varies by OS.
So, add a configure check for it.
Also, renamed showFile to filePathToString
Internally, the filenames are stored as un-decoded unicode.
I tried decoding them, but then haskell tries to access the wrong files.
Hmm.
So, I've unhappily chosen option "B", which is to decode filenames before
they are displayed.
