system-posix-redirect.cabal: reformat
system-posix-redirect.cabal: use per-file {-# LANGUAGE #-} pragmas
system-posix-redirect.cabal: ghc-options: -Wall
System.Posix.Redirect: import Control.Exception unused

∙ Also take the opportunity to rearrange imports.
System.Posix.Redirect: use Control.Monad.void