Use drupal_exit() rather than exit() in PuSH/FeedsHTTPFetcher callbacks

The code change for #870556: PuSH: verification fails the first time moved PuSH feeds subscriptions into hook_exit(). In order for this to be properly executed during a PuSH workflow (i.e. if you had a PuSH feed that created more PuSH feeds) the feeds module should use drupal_exit() rather than exit().

If there's a specific reason exit() was used rather than drupal_exit() I would advocate for invoking feeds_exit() directly in these cases to avoid issues with new PuSH feeds.

Patch to follow.