Enable altering of service whitelist

Problem/Motivation

I have a need to modify the CAS whitelist depending on the user's roles/permissions to block or allow certain services from being access through CAS.

Proposed resolution/API changes

Add new CAS API hook, cas_server_service_whitelist_alter, to allow for altering of whitelist.

In the provided patch, the hook is called during _cas_server_check_service_whitelist. cas_server_service_return is also lightly modified to ensure that the service is always double-checked against the whitelist, even if the check was done beforehand.