OvrInit procedure

  DECLARATION:  OvrInit(FileName : String)

      PURPOSE:  Initializes the overlay managerand opens the overlay file.

         UNIT:  Overlay

      REMARKS:  If Filename does not specify a drive or subdirectory path
                the overlay manager will search in the current directory, in
                the directory where the .EXE file exists (if running under
                DOS v3.xx), and in the directories specified in the DOS PATH
                environment variable.