daemon error handling: Define a new function reply_with_perror_errno.

daemon error handling: Define a new function reply_with_perror_errno.

This allows you to save the errno from a previous call and
pass it to reply_with_perror.

For example, original code:

  r = some_system_call ();
  err = errno;
  do_cleanup ();
  errno = err;
  if (r == -1) {
    reply_with_perror ("failed");
    return -1;
  }

can in future be changed to:

  r = some_system_call ();
  err = errno;
  do_cleanup ();
  if (r == -1) {
    reply_with_perror_errno (err, "failed");
    return -1;
  }