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.