(nntp-retrieve-groups): Ditto for groups.
(nntp-retrieve-articles): Ditto for articles.
(*): Replaced nntp-possibly-change-group calls to
- nntp-with-open-group forms in all, but one, occurrance.
+ nntp-with-open-group forms in all, but one, occurrence.
(nntp-accept-process-output): Bug fix. Detect when called with
null process.