Sidebar handling, revisited

For posts that are really old, or posts that have a value of "default," the option in the post edit screen dropdown will be the single post sidebar. For new posts, "none" will be the pre-selected value. For other posts, they'll get whatever's stored in the database.

On the front end, if a post is set to 'default', it'll be handled as if it were set to 'sidebar-single'. Unlike other custom values, 'sidebar-single' will fall back to displaying the 'sidebar-main' region if sidebar-single is empty.  Other custom values will just try to show the custom region.

Really old posts that don't have any value set at all will behave how they always have, also getting 'sidebar-single' if active or 'sidebar-main' if not.

I *think* this covers everything.