Do not apply 'max_failures' to random-exponential schedules.

  o Major bugfixes (download scheduling):

    - When using an exponential backoff schedule, do not give up on

      dowloading just because we have failed a bunch of times.  Since

      each delay is longer than the last, retrying indefinitely won't

      hurt. Fixes bug 20536; bugfix on 0.2.9.1-alpha.

download_status_is_ready(download_status_t *dls, time_t now,

                         int max_failures)

{

  if (dls->backoff == DL_SCHED_DETERMINISTIC) {

    /* Deterministic schedules can hit an endpoint; exponential backoff

     * schedules just wait longer and longer. */

    int under_failure_limit = (dls->n_download_failures <= max_failures

                               && dls->n_download_attempts <= max_failures);

  return (under_failure_limit && dls->next_attempt_at <= now);

    if (!under_failure_limit)

      return 0;

  }

  return dls->next_attempt_at <= now;

}

static void download_status_mark_impossible(download_status_t *dl);