Introduce FSFS expert tool 'svnfsfs'.  It is intended to grow various
FSFS-specific commands in the future - like recreating repcache.db etc.

For now, it provides two commands to read (dump) and write (load)
format 7 index information.  With these, corrupted repositories can
be manipulated / fixed by hand or script.

The former fsfs-stats tool becomes a sub-command as well.

* build.conf
  (svnfsfs): Define new target for the new tool.
  (__ALL__): Add new tool as dependency.
  (__MORE__): Remove fsfs-stats dependency.
  (fsfs-stats): Drop superseded target.

* subversion/include/svn_error_codes.h
  (SVN_ERR_FS_UNSUPPORTED_TYPE,
   SVN_ERR_INVALID_INPUT): Declare new error types to be used by the
                           new tool.

* tools/server-side/fsfs-stats.c
  (): Removed, non-boilerplate sources moved to ./svnfsfs/stats-cmd.c

* tools/server-side/svnfsfs
  (): New directory for the new tool; ignore new binary.

* tools/server-side/svnfsfs/svnfsfs.h
  (): New file containing the tool globals.

* tools/server-side/svnfsfs/svnfsfs.c
  (): Boilerplate command line interface, command and parameter definition.

* tools/server-side/svnfsfs/dump-index-cmd.c
  (): New 'dump-index' sub-command implementation.

* tools/server-side/svnfsfs/load-index-cmd.c
  (): New 'load-index' sub-command implementation.

* tools/server-side/svnfsfs/stats-cmd.c
  (): New 'stats' sub-command implementation.  Code taken from former
      stand-alone tool. 


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1594860 13f79535-47bb-0310-9956-ffa450edef68