Code review request: 6355584: Introduce constrained Kerberos delegation

[Weijun Wang]

Hi All

Please review
   http://cr.openjdk.java.net/~weijun/6355584/webrev.00/

This enables 2 changes:

1. As an initiator, you can call

   ((ExtendedGSSCredential)cred).impersonate(other)

to impersonate a client.

2. As an acceptor, context.getDelegCred() can still return a constrained 
delegated credential even if the initiator has not called 
context.requestCredDeleg(true) to enable traditional delegation.

These are implemented with MS's S4U2self and S4U2proxy extensions to 
Kerberos 5.

Thanks
Max