Rename procedure

  DECLARATION:  Rename(var F; NewName : String);

      PURPOSE:  Renames an external file

         UNIT:  System

      REMARKS:  F may be a file variable of any file-type, and NewName is
                a string-type expression.  The external file associated with
                F will be renamed to NewName.  All future operations on F
                will be operated on the external file with the new name.

                NOTE: With {$I-}, errors are returned through IOResult