RFR 8043071: Expose session key and KRB_CRED through extended GSS-API

[Wang Weijun]

Updated webrev at

   http://cr.openjdk.java.net/~weijun/8043071/webrev.01/

All your suggestions accepted, plus

- New test on new classes added into KerberosHashEqualsTest.java

- A duplicate test/sun/security/krb5/auto/KerberosHashEqualsTest.java removed

- I don't know why KerberosKey allows a null principal, but anyway update its toString() to avoid NPE

- KerberosTicket.toString() now returns "Destroyed KerberosTicket" instead of throwing an ISE, to be consistent with other classes

Thanks
Max