Make svn_fs_lock_target_t opaque in the public API so that any
future changes to the target parameters are easier.

* subversion/include/svn_fs.h
  (struct svn_fs_lock_target_t): Change to forward declaration.
  (svn_fs_lock_target_create, svn_fs_lock_target_set_token): New.

* subversion/libsvn_fs/fs-loader.h
  (struct svn_fs_lock_target_t): Define.

* subversion/libsvn_fs/fs-loader.c
  (svn_fs_lock_target_create, svn_fs_lock_target_set_token): New.

* subversion/libsvn_repos/fs-wrap.c
  (svn_repos_fs_lock_many): Use svn_fs_lock_target_set_token.
  (svn_repos_fs_lock): Use svn_fs_lock_target_create.

* subversion/libsvn_ra_local/ra_plugin.c
  (svn_ra_local__lock): Use svn_fs_lock_target_create.

* subversion/svnserve/serve.c
  (lock_many): Use svn_fs_lock_target_create.

* subversion/tests/libsvn_fs/locks-test.c
  (lock_multiple_paths): Use svn_fs_lock_target_create.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1584872 13f79535-47bb-0310-9956-ffa450edef68