* gnus-agent.el (gnus-agent-read-agentview): Removed support for
old file versions.
(gnus-group-prepare-hook): Removed function that converted list
form of gnus-agent-expire-days to group properties.
* gnus-int.el: Autoload gnus-agent-regenerate-group.
(gnus-request-accept-article): Re-indented.
* gnus-start.el (gnus-convert-old-newsrc): Registered new
converters to handle old agent file formats. Added logic for a
"backup before upgrading warning".
(gnus-convert-mark-converter-prompt): Developers can mark
functions as needing (default), or not needing,
gnus-convert-old-newsrc's "backup before upgrading warning".
(gnus-convert-converter-needs-prompt): Tests whether the user
should be protected from potentially irreversable changes by the
function.
* legacy-gnus-agent.el (): New. Provides converters that are only
loaded when gnus-convert-old-newsrc needs to call them.