* lisp/gnus/gnus-sum.el: Prepare for lexical-binding

* lisp/gnus/gnus-sum.el: Prepare for lexical-binding

Add defvars for all the gnus-tmp-*.
(gnus-summary-make-local-variables): Move let binding to avoid setq.
(gnus-set-global-variables): Use dolist.
(gnus-summary-from-or-to-or-newsgroups, gnus-summary-insert-line)
(gnus-summary-insert-dummy-line): Avoid dynbind args.
(gnus-build-old-threads): Remove unused var 'id'.
(gnus-nov-parse-line): Remove unused var 'buffer'.
(gnus-thread-header): Prepare it for a lexbind world.
(gnus-adjust-marked-articles): Remove unused var 'marks'.
(gnus-mark-xrefs-as-read): Remove unused var 'idlist'.
(gnus-summary-display-article): Erase&widen before mm-enable-multibyte.
(gnus-summary-better-unread-subject): Remove unused var 'score'.
(gnus-summary-find-matching): Remove unused var 'd'.
(ps-right-header, ps-left-header, shr-ignore-cache): Declare vars.
(gnus-summary-idna-message, gnus-summary-morse-message)
(gnus-summary-sort-by-original):
Fix interactive spec since we don't actually use any prefix arg.
(gnus-summary-move-article, gnus-read-move-group-name): Use user-error.
(gnus-summary-move-article): Use dolist.
(gnus-summary-edit-article): Fix unquoting.
(gnus-summary-highlight-line-0, gnus-summary-highlight-line):
Declare dynbind vars documented in gnus-summary-highlight.