patch applied (ghc): fix exit code of ghci -e " return ()"

[Ian Lynagh]

Thu May  3 14:25:14 PDT 2007  Bertram Felgenhauer <int-e at gmx.de>
  * fix exit code of ghci -e "return ()"
  This should result in successful program termination. Not doing this
  breaks checking the exit code of  runghc Setup.lhs build  for example.

    M ./compiler/ghci/InteractiveUI.hs -1 +6