Switch the FS open and basic file access logic in fsfs-stats
from local logic to svn_fs and svn_fs_fs, respectively.

* build.conf
  (fsfs-stats): Add FS and FSFS libs as dependencies.

* tools/server-side/fsfs-stats.c
  (revision_info_t): Make start and end offsets apr_off_t for
                     better compatibility with FSFS APIs.
  (fs_t): Add svn_fs_t as sub-element and remove all redundant
          members from the top-level struct.
  (get_pack_folder,
   rev_or_pack_file_name): Drop.
  (open_rev_or_pack_file): Call FSFS API.
  (read_manifest,
   read_format,
   read_number): Drop.
  (fs_open): Call FS API and simplify format check.
  (find_representation,
   get_rep_content): We always start at rev 0, thus drop the
                     START_REVISION arithmetics.
  (read_pack_file): Use the FSFS API to determine offset
                    ranges in pack files.
  (read_revision_file,
   read_revisions): Use the info from the FSFS API struct
                    that we removed from from the local fs_t.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1553337 13f79535-47bb-0310-9956-ffa450edef68