Second attempt at a new authentication handler for JavaHL.

* subversion/bindings/javahl/src/org/apache/subversion/javahl/callback/AuthnCallback.java
  (AuthnCallback): More or less completely reworked.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1598265 13f79535-47bb-0310-9956-ffa450edef68