distribute dummy on Add in _osborne

Rather than make the osborne and the osbornei roundtrip by dividing
by d in osbornei to undo the multiplying in osborne, the simple
replacement of d with 1 is used instead.

If the dummy is not distributed, arguments that looked like the addition
of two terms no longer appear so and some transformations (like TR12)
then fail.
What's this?  We have such a test (see test failure in #7788).