Prepare the 'svnfsfs dump-index' code for being split up into UI,
FSFS internal logic and a private interface in between.

This patch introduces the interface structure but does not move
any code around.  We add a per index record callback function
that the UI can implement to show the contents while the actual
query would be run in FSFS.

* subversion/svnfsfs/dump-index-cmd.c
  (svn_fs_fs__dump_index_func_t): The new callback type.
  (dump_index_entry): The former record display part of dump_index.
  (svn_fs_fs__dump_index): The suggested FSFS private API function.
                           It contains the logic of the previous
                           dump_index but expects the FS to be passed
                           in instead of being opened locally.  Invoke
                           the callback instead of displaying data.
                           Finally, add cancellation support.
  (dump_index): Stripped down to opening the FS, writing the header
                line and then invoking the new function to drive
                the index processing.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632855 13f79535-47bb-0310-9956-ffa450edef68