Move the FSFS index load logic into FSFS.

* subversion/include/private/svn_fs_fs_private.h
  (svn_fs_fs__load_index): Declare new API, taken from load-index-cmd.c.
                           Extend docstring.

* subversion/libsvn_fs_fs/load-index.c
  (): New file.  Update #includes.
  (svn_fs_fs__load_index): Moved here from load-index-cmd.c.

* subversion/svnfsfs/load-index-cmd.c
  (): Update #includes.
  (svn_fs_fs__load_index): Remove here.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632868 13f79535-47bb-0310-9956-ffa450edef68