Use format_plural() on node delete confirmation page

Go to /admin/content/node, check one node, and choose delete from the update menu. The header of the subsequent page says "these items" and the submit button says "delete all." See below:

Here's a patch that checks whether the number of nodes being deleted is greater than one. If it's greater than one, the header says what it already does. If it's one, the header refers to the item in singular form.

This patch changes the "Delete all" text on the submit button to just "Delete," which I think expresses the same thing given the warning in the header, and it also avoids having an awkward use of format_plural(). If people absolutely love "Delete all" I can revise it to use format_plural() like the the header does in this patch.

The indentation here is awkward. Let me know if/how it can be improved.

A similar change would be needed for the submit handler, but I'm not sure if it would be better to use format_plural() or just two separate drupal_set_message() calls within an if block. I'll wait for feedback on that.

(Is there a tag for textual/language work?)