RFR: 8272162: S4U2Self ticket without forwardable flag

[Weijun Wang]

The S4U2proxy extension requires that the service ticket to the first service has the forwardable flag set, but some versions of Windows Server do not set the forwardable flag in a S4U2self response and accept it in a S4U2proxy request.

There are 2 commits now. The 1st is a refactoring that sends more info into the methods (Ex: `KdcComm::send(byte[])` -> `KdcComm::send(KrbKdcReq)`, and `Ticket` -> `Credentials` in multiple places) so that inside `KdcComm::send` there is enough info to decide how to deal with various errors. The 2nd is the actual fix to this issue, i.e. ignore the flag and retry another KDC.

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Commit messages:
 - TGT needs not to be forwardable in S4U2self request
 - address martin's comments
 - Merge
 - also a security property
 - a system property, do not care where ticket is from, more renames
 - move KDCReq::encoding to KrbKdcReq::obuf, no more ibuf in KrbTgsReq
 - a type label for credentials, encoding in KrbKdcReq, and some renames
 - implement the change
 - 8272162: S4U2Self ticket without forwardable flag

Changes: https://git.openjdk.java.net/jdk/pull/6082/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6082&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272162
  Stats: 413 lines in 17 files changed: 218 ins; 38 del; 157 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6082.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6082/head:pull/6082

PR: https://git.openjdk.java.net/jdk/pull/6082