Get kerberos authentication working again.

This revision requires internal changes to the application's web.xml and jboss-web.xml which is not ideal.
Fix Kerberos security problems.

User name not correctly retrieved from external context.
Add new klogin page specifically for kerberos authentication.
Still not single war compatible.
Add a central Login module which will delegate to the appropriate auth dependent module.
Create a central Login module that delegates to the others.