New domain preference: SOGoIMAPCASServiceName

Set this to the service name expected by the CAS server if it differs
from SOGoIMAPServer. This is useful to request a CAS ticket for service
imap://imap.domain.com while connecting through imapproxy on imap://127.0.0.1:1143

SOGoDAVAuthenticator is not updated since it doesn't really use the imap code.