JASPI Fixes

Resending proposed changes after rebasing to eliminate rogue commits in previous pull request.

This fixes a few issues with JASPI:
* Pre-emptive authetication (AUTH_SUCCESS & null UserPrinciplal
handling) (spec 3.8.3.1)
* Always call secureResponse (spec 2.5.1.2 Point(3))
* Properly fill org.jboss.security.SecurityContext by using
org.jboss.security.auth.callback.JASPICallbackHandler, so that EJB
propgation has a chance to work