svds.m: Overhaul code while fixing bug #29721. Return smallest singular values if sigma == 0 (Bug #29721). Avoid calculating U and V matrices unless requested. Correctly handle zero matrix input Improve documentation string.

      subroutine xsdot (n, dx, incx, dy, incy, retval)
      real ddot, dx(*), dy(*), retval, sdot
      integer n, incx, incy
      retval = sdot (n, dx, incx, dy, incy)
      return
      end