Truncate procedure

  DECLARATION:  Truncate(Var FileVar);

      PURPOSE:  Truncates the file size at the current file position

         UNIT:  System

      REMARKS:  FileVar is a File-Type variable of any type.  All
                records past the current file position of FileVar are
                lost as the current file position becomes the end-of-file
                (EOF(FileVar) is TRUE).

                NOTE:  Errors are returned through IOResult.