RFR 8005819: Support cross-realm MSSFU

[Martin Balao]

Hi,

Webrev.02:

 * http://cr.openjdk.java.net/~mbalao/webrevs/8005819/8005819.webrev.02/

Changes:

 * No need to create a new sname PrincipalName in
CredentialsUtil::handleS4U2ProxyReferral as it's not mutable.

Regards,
Martin.-