SwapVectors procedure

  DECLARATION:  SwapVectors

      PURPOSE:  Swaps interrupt vectors

         UNIT:  DOS

      REMARKS:  SwapVectors will swap the contents of the SaveIntXX
                pointers in the System Unit with the current interrupt
                vectors.  You should call this before and after calling
                the Exec procedure to insure that the Exec'd program does
                not use any interrupt handlers installed by the current
                process and vice-versa.