RFR 7176784: Windows authentication not working on some computers

[Chris Hegarty]

This is a trivial (2 line) change to increase the size of a native 
buffer being passed to InitializeSecurityContext. We have seen reports 
of InitializeSecurityContext failing with SEC_E_INSUFFICIENT_MEMORY, 
given the needed buffer size depends on Windows domain size, DNS domain 
name, client computer name size, server computer size, user name size, etc.

http://cr.openjdk.java.net/~chegar/7176784/webrev.00/webrev/

This fix is deliberately conservative as I would like to get it into 7u6 
also. It may be that this area could do with some refactoring and better 
error checking in the future, but I'd like to keep this change as is to 
address the specific issue.

-Chris.