Allow extra_fields to have configure links

Modules can expose their custom node data for re-ordering using hook_content_extra_fields, but there's no way to add a CCK-style 'configure' or 'remove' link to the rows. This four-line patch makes that possible, and preserves backwards compatibility with existing hook implementations.

For modules like views_attach, this means that the "manage fields" entry for an attached view could link directly to the Views configuration screen for the display.