* lisp/net/newst-backend.el: Use lexical scoping and fix warnings

* lisp/net/newst-backend.el: Use lexical scoping and fix warnings

(newsticker-stop, newsticker-get-all-news)
(newsticker--decode-rfc822-date, newsticker--lists-intersect-p)
(newsticker--update-process-ids, newsticker--cache-read)
(newsticker-opml-export, newsticker--run-auto-mark-filter)
(newsticker--do-run-auto-mark-filter): Use dolist.
(newsticker--insert-bytes): New function, to avoid string-to-multibyte.
(newsticker--get-news-by-funcall, newsticker--get-news-by-url-callback)
(newsticker--image-download-by-url-callback): Use it.
(newsticker--parse-rss-0.91, newsticker--parse-rss-0.92):
Remove unused var `pub-date`.
(newsticker--parse-generic-feed): Remove unused var `old-item`.
(newsticker--parse-generic-items): Use dolist and let rather than mapc
and setq.
(newsticker--image-download-by-url-callback): Simplify boolean expression.