Agent Customization check-in

[Kevin Greiner <kgreiner@xpediantsolutions.com>]

Just to let everyone know, I just finished a rather large check-in. 

If you download individual files, the following files should be
downloaded together because their modifications are cross-dependent.

  gnus-agent.el, gnus-art.el, gnus-cus.el, gnus-int.el

Also, gnus-start.el is dependent on a change in gnus-util.el.

The new features are:
* Agent parameters are now merged with group/topic parameters.  This
  means that you can customize agent behavior at the individual group
  level.
* Agent categories (which are still supported) now have a
  customization buffer.  The 'e' command in the category buffer will
  display it.
* The precidence rule for parameters is in decending order: group
  parameter, topic parameter, category parameter, defined constant.
* A new agent parameter has been added.  It enables/disables agent
  expiration per individual group.
* The gnus-agent-cache feature, which did not appear to function per
  the documentation, has been re-implemented.  When gnus-agent-cache
  is cleared (i.e. nil), the plugged agent will always fetch from the
  server.
* You can now configure gnus to write .newsrc.eld directly to disk.
  It is considerably slower than saving a temporary buffer but can
  prevent memory exhaustion errors.

Kevin