In FSFS, always use the same function to read the 'current' file.

Apart from the consistency aspect, this no longer lets atoi() mask
'current' file corruptions.  Recovery must be adopted to this.

* subversion/libsvn_fs_fs/fs_fs.c
  (get_youngest): Change signature because we will need the format
                  info as well - not just a repo path.  Call the util
                  function to read and interpret the 'current' file.
  (with_some_lock_file,
   svn_fs_fs__open,
   svn_fs_fs__youngest_rev,
   svn_fs_fs__ensure_revision_exists): Update callers.

* subversion/libsvn_fs_fs/fs.c
  (fs_open_for_recovery): Make sure, a valid 'current' file is available
                          for repository recovery.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1629854 13f79535-47bb-0310-9956-ffa450edef68