Make `-Wall` clean and drop `-fno-warn-unused-imports`

CPP conditional code sections have been carefully taken into account
while performing this warning cleanup. Most `OPTIONS_GHC` declarations
could be dropped (now only a lonely `{-# OPTIONS_GHC -fno-cse #-}`
remains in `Posix.Signals`).

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>