make adding data more flexible in deletion api

with this small change, drupal_delete_add_form_elements() becomes a far more flexible function. it was originally written just to help handle building the multiple delete confirm screens, but this tweak enables it to be used generally anywhere that a module would like to inject a piece of a form into a confirm screen. great use case is for putting a message into the confirm form directly from hook_nodeapi 'delete' op.

initial testing is good. i'll run a few more tests to make sure it doesn't break any current functionality...