OvrInitEMS procedure

  DECLARATION:  OvrInitEMS

      PURPOSE:  Initializes the overlay file into EMS if possible

         UNIT:  Overlay

      REMARKS:  An EMS driver and sufficient memory must be detected.  If
                so, all overlays are loaded into EMS and the overlay file is
                closed.  OvrInitEMS also installs an exit routine which will
                deallocate the EMS memory upon program termination.

                The EMS driver must conform to the LIM (Lotus, Intel,
                Microsoft) Expanded Memory Specification (EMS).

                Errors are reported through OvrResult; See the error
                messages section for details about OvrResult.