API: check_permission and check_permissions flags

API calls use both check_permission (in contact context to allow performing dedupe checks against the whole contact database, even if the caller is, for example, anonymous) and check_permissions to side-step the API permission checking.

This is most confusing and we need to find a way to clear up the confusion; tentative ideas:

1. Settle on check_permissions and do
if (!empty($params['check_permission'])) $params['check_permissions'] = $params['check_permission'];
in the API wrapper and all other places that end up calling dedupe.

2. Teach dedupe to honour check_permissions instead of check_permission.

3. Deprecate check_permission as of 4.2 at the earliest.