Improving support for upgrading gnus

[Kevin Greiner <kgreiner@xpediantsolutions.com>]

My most recent post included a rewritten gnus-convert-old-newsrc.  The
new function should make it possible to move legacy support out of
individual gnus source files and into a separate source tree.  The
goal is to make gnus leaner, faster, and easier to maintain without
sacrificing support for the user community.

Here are the key new features in gnus-convert-old-newsrc.

1) The converter functions no longer need to be defined at the time
   gnus-convert-old-newsrc is compiled/loaded.  Unbound converters may
   be loaded, at run-time, from a location specified in the conversion
   table.

   - I'd like to see a consensus for moving legacy conversion code
     into a directory named legacy.  The code in this directory will
     be included in the gnus build and installation but it will only
     be loaded when upgrading an old installation to the current 
     version.

2) Multiple converters are sequentially invoked to successively
   convert very old versions into the current version.

   - This should make it easier to write converters since you only
     have to be concerned with the version immediately before the
     version originating the converter.

3) All conversions, whether a single trivial update or a massive
   sequence, must be authorized by the user.

   - This alters the user to the possibility that they are taking an
     irreversible step forward and points out that they should first
     make backups.

Personally, I'd like to clean up gnus-agent.el by moving support for
history files, uncompressed agentview files, the old regexp list in
gnus-agent-expire-days, etc. in legacy/gnus-agent.el.  I fairly 
certain other parts of gnus would also benefit.

So, comments anyone?

Kevin