Merge bugfixes done in Gnus trunk

Merge bugfixes done in Gnus trunk

Those changes fix only the bugs having appeared in the bug list.
Many other Gnus changes not yet merged to Emacs are in:
   ftp://ftp.jpl.org/pub/tmp/MaGnus-to-Emacs.patch
(or http://www.jpl.org/ftp/pub/tmp/MaGnus-to-Emacs.patch)

2012-06-11 Lars Magne Ingebrigtsen <larsi@gnus.org>
* gnus.texi (Group Timestamp): Mention where to find documentation for
  the `gnus-tmp-' variables (bug#11601).
2012-04-14 Wolfgang Jenkner <wjenkner@inode.at>
* gnus-agent.el (gnus-agent-retrieve-headers): Recalculate the range of
  articles when fetch-old is non-nil (bug#11370).