Admin adding anonymous users to events- d6

This is a request and patch for the same functionality requested in #215211: Admin adding anonymous users to events but is intended for Drupal 6.x.

The admin form was modified to check if anonymous users can signup for content. If they can, it replaces the user form field with a fieldset that contains both the username and an email address field. The fieldset's description is used to explain to the administrator that they need to enter a username or an email address.

If the fieldset is displayed a new validation function 'signup_form_validate_admin_anon' (perhaps this could be named better?) is called which makes sure that only a username or email address was entered and then calls the existing 'signup_form_validate_username' or 'signup_form_validate_anon'.

The submit handler 'signup_form_submit' was modified from using an isset on the signup_username to !empty in the event that both username and email address fields exist.