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)

2012-06-07 Lars Magne Ingebrigtsen <larsi@gnus.org>
* gnus-msg.el (gnus-msg-mail): Warn the user about Gnus not running
  (bug#11514).
2012-04-10 Lars Magne Ingebrigtsen <larsi@gnus.org>
* gnus-msg.el (gnus-summary-cancel-article): See what From header we
  would have gotten if we posted to the group, and use that to compare
  against the message we want to cancel (bug#10808).