don't duplicate field names on the same form

For writing tests, etc, it would be easier to not have fields changing names. On the "Add filed" form, if there are existing fields, two forms on the same page are trying to use 'field_name'. It's fine for FAPI, but makes it harder to write tests, since the ID to post to will change depending on the # of existing fields, and the fields already added to the given content type.

Attached patch is trivial and tested.