Add an explicit baton for the tunnel handling callbacks,
and make parameter names more consistent.

* subversion/include/svn_client.h (svn_client_ctx_t): New member tunnel_baton.
* subversion/include/svn_ra.h
  (svn_ra_callbacks2_t): New member tunnel_baton.
   Renamed open_tunnel to open_tunnel_func,
   and close_tunnel to close_tunnel_func.
  (svn_ra_open_tunnel_func_t, svn_ra_close_tunnel_func_t):
   Rename parameter tunnel_baton to tunnel_context.
   Rename parameter open_baton to tunnel_baton,
   to match the callbacks structure.

* subversion/libsvn_client/ra.c (svn_client__open_ra_session_internal):
   Update for renamed callback struct members and init the
   tunnel_baton in the RA callbacks.
* subversion/libsvn_ra_svn/client.c
  (tunnel_data_t, close_tunnel_cleanup, open_session, ra_svn_open):
   Update for renamed callback struct members.
* subversion/tests/libsvn_ra/ra-test.c: Likewise.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1531777 13f79535-47bb-0310-9956-ffa450edef68