Clean up Posix monad transformers.

[judah]

Clean up the code around PosixT.

While I'm at it, remove this warning:

System/Console/Haskeline/Backend/Posix.hsc:255:0:
    Warning: Definition but no type signature for `runPosixT'
             Inferred type: runPosixT :: forall r (m :: * -> *) r1 a.
                                         (Monad m) =>
                                         r1 -> r -> ReaderT r1 (ReaderT r m) a -> m a

[judah]

That warning has been suppressed.