a unification for the sendfile syscall

c_sendfile :: Fd -> Fd -> COff -> CSize -> IO CSsize

#if defined(LINUX)
c_sendfile ofd ifd o c = do
  alloca $ \off -> do
  poke off o
  r <- c_sendfile_linux ofd ifd off c
  return r
#endif

#if defined(DARWIN)
c_sendfile ofd ifd o c = do
  alloca $ \sbytes ->
    do poke sbytes c
       c_sendfile_darwin ifd ofd o sbytes 0 0 
#endif

#if defined(FREEBSD)
c_sendfile ofd ifd o c = do
  alloca $ \sbytes ->
    do poke sbytes c
       c_sendfile_freebsd ifd ofd o c 0 sbytes 0
#endif