Begin replacing the far-too-simplified authentication callback in JavaHL;
it's really hard to use the current one in any meaningful way in a GUI.

[in subversion/bindings/javahl]
* src/org/apache/subversion/javahl/callback/UserPasswordCallback.java
  (UserPasswordCallback): Mark as deprecated.
* src/org/apache/subversion/javahl/callback/AuthnCallback.java
  (AuthnCallback): Define the new interface.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1597758 13f79535-47bb-0310-9956-ffa450edef68