krb5_child: send back the client principal
    
    In general Kerberos is case sensitive but the KDC of Active Directory
    typically handles request case in-sensitive. In the case where we guess
    a user principal by combining the user name and the realm and are not
    sure about the cases of the letters used in the user name we might get a
    valid ticket from the AD KDC but are not able to access it with the
    Kerberos client library because we assume a wrong case.
    
    The client principal in the returned credentials will always have the
    right cases. To be able to update the cache user principal name the
    krb5_child will return the principal for further processing.