RFR 8005819: Support cross-realm MSSFU

[Martin Balao]

Hi,

I'd like to propose Webrev.01 for 8005819 [1]:

 * http://cr.openjdk.java.net/~mbalao/webrevs/8005819/8005819.webrev.01/

New changes:

 * Avoid re-checking S4U2Proxy and S4U2Self status in each referral
request (do this only once in "CredentialsUtils::serviceCreds")

 * Other minor improvements (code readability introducing S4U2Type, do
not propagate client alias for impersonation TGSs, debugging output)

Testing: no regressions found in jdk/sun/security/krb5.

Thanks,
Martin.-

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[1] - https://bugs.openjdk.java.net/browse/JDK-8005819