Allow anonymous users to opt-out to authenticated newsletters

Subscribe and unsubscribe should not share the same permissions. A user should be able to unsubscribe from a newsletter without loging in. The simplest way is to separate the menu like so:

function simplenews_menu() {
	...
	$items['newsletter/confirm/remove'] = array(
		'title' => 'Confirm newsletter subscriptions',
		'type' => MENU_CALLBACK,
		'page callback' => 'simplenews_confirm_subscription',
	    'page arguments' => array(2),
		'access arguments' => array('access content'),
		'file' => 'simplenews.subscription.inc',
	);

Of course, the simplenews module should have a different permission for unsubscribing.

I would prefer to be able to manage this setting per newsletter, but for now, this patch does the work.

Thank you.