* lisp/gnus/nnmh.el (nnmh-newsgroup-articles): Declare var

* lisp/gnus/nnmh.el (nnmh-newsgroup-articles): Declare var

Reported by Barry Fishman <barry@ecubist.org>.

Along the way, I checked other variables which are similarly let-bound
to nil and then read with any intervening assignment, which found
another similar case of missing `defvar`s plus a bit of dead code.

* lisp/gnus/gnus-kill.el (gnus-apply-kill-file-internal):
Remove constant nil var `beg`.
* lisp/gnus/gnus-search.el (gnus-search-query-parse-kv):
Remove constant nil var `return`.
* lisp/gnus/gnus-start.el (gnus-ask-server-for-new-groups):
Remove constant nil var `group`.
(gnus-killed-assoc, gnus-marked-assoc, gnus-newsrc-assoc): Declare vars.
* lisp/gnus/gnus-sum.el (gnus-compute-read-articles):
Remove constant nil var `first`.
* lisp/gnus/nnbabyl.el (nnbabyl-request-accept-article):
Remove constant nil var `beg`.
* lisp/gnus/nnfolder.el (nnfolder-possibly-change-group):
Remove constant nil var `inf`.
* lisp/gnus/nnrss.el (nnrss-request-article):
Remove constant nil var `err`.