Follow-up on r1632694: Commit the missing file changes.

* subversion/svnfsfs/svnfsfs.h
  (check_cancel): Declare global cancellation callback to be accessible
                  to all sub-commands.

* subversion/svnfsfs/svnfsfs.c
  (check_cancel): New global cancellation callback. Copied over from
                  svnadmin.c

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632695 13f79535-47bb-0310-9956-ffa450edef68