* gnus-start.el (gnus-convert-old-newsrc): Registered new

	* 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.