sf_scrntodisk         Screen save to disk
------------------------------------------------------------------------------

    procedure sf_scrntodisk prototype
      parameters const char file_name,;
                 value int lr,;
                 value int lc,;
                 value int rr,;
                 value int rc

    By utilizing the sf_savescrn() and sf_restscrn() functions, we have added
    these two functions which allow the screen to be saved/restored to/from
    disk instead of memory.