Authenticated Guest Mode

[davelebbing]

Feature description:

1. User logs in with XCreds at the cloud login window or AD login. XCreds verifies that cloud password or against the domain controller.
2. If prefs are set to guest mode, the local directory service account is updated for the details from the cloud or AD user account info.
3. The user logs in and the keychain is set with the login username and password. Same for the user account. That info is also used to get a kerberos ticket.
4. When the user logs out, the home directory and keychain is removed.

@tperfitt see ticket info.