This patch mostly deals with the fact that I read multiple newsgroups
at once (setq gnus-single-article-buffer nil).  In fact, I don't
understand how anyone can survive with the default.

Three changes in one patch:

1. Using picons sets the mode of *Article* to 'gnus-picons-mode.
   However, `article-update-date-lapsed' only checks for for
   'gnus-article-mode.

2. The variable gnus-current-header is only set for gnus-summary-mode.
   Therefore, all *Article* buffers are getting the X-Sent of
   whichever article I last hit SPC in.

3. `walk-windows' only walks in the current frame.  We really want to
   walk in all visible frames.  Otherwise, side-by-side newsgroup
   reading loses.