Move the FSFS index dump logic into FSFS.

* subversion/include/private/svn_fs_fs_private.h
  (svn_fs_fs__dump_index_func_t,
   svn_fs_fs__dump_index): Declare new API, taken from dump-index-cmd.c.

* subversion/libsvn_fs_fs/dump-index.c
  (): New file.  Update #includes.
  (svn_fs_fs__dump_index): Moved here from dump-index-cmd.c. Replace
                           the INDEX_BLOCK_SIZE reference with info
                           taken from the FS.

* subversion/svnfsfs/dump-index-cmd.c
  (): Update #includes.
  (svn_fs_fs__dump_index_func_t,
   svn_fs_fs__dump_index): Remove here.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632857 13f79535-47bb-0310-9956-ffa450edef68