Revision 1658123

[Reverted in r1658150]

Expose the Gnome keyring unlock prompt to the JavaHL C++ code.
(Exposing it to JavaHL itself is a different step where brane might
 have better ideas)

* subversion/bindings/javahl/native/OperationContext.cpp
  (OperationContext::getAuthBaton):
     Hook gnome prompt if we have a prompter implementation.

* subversion/bindings/javahl/native/Prompter.cpp
  (Prompter::get_gnome_keyring_unlock,
   Prompter::gnome_keyring_unlock_prompt): New function.

* subversion/bindings/javahl/native/Prompter.h
  (Prompter::get_gnome_keyring_unlock,
   Prompter::gnome_keyring_unlock_prompt): New function.

* subversion/libsvn_auth_gnome_keyring/gnome_keyring.c
  (ensure_gnome_keyring_is_unlocked): Allow prompt func to not provide a
    password, similar to our other credential callbacks. This makes it easier
    for bindings to conditionally handle the prompt.