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.

SoObjects/SOGo/SOGoDomainDefaults.m

@@ -123,6 +123,11 @@
   return [self stringForKey: @"SOGoSieveServer"];
 }
 
+- (NSString *) imapCASServiceName
+{
+  return [self objectForKey: @"SOGoIMAPCASServiceName"];
+}
+
 #warning should be removed when we make use of imap namespace
 - (NSString *) imapAclStyle
 {