Exclude author from sitemap

Fix for issue: #972

- Individual users can be excluded from sitemaps (checkbox addition in user edit page)
- User roles can be excluded from sitemaps (on the sitemap settings page)
Yes, don't use `get_editable_roles()` ;-) Editable roles is generally filtered based on the permissions the current user has to edit other users, so that's not the list you want anyway.

Try something along the lines of this:
```php
global $wp_roles;
 
if ( ! isset( $wp_roles ) ) {
	$wp_roles = new WP_Roles();
	$roles = $wp_roles->get_names();
}
```

Oh and enrich_defaults is (re-)run on the init hook at prio 99 to ensure "things" are available.