Equality: less automatic simplification and fix 'Eq(p, 0)' for p positive

update: no longer on top of #7959 and #7838.
> If is_real is None, we need to let .equals(0) try to figure it out...

The whole "dif" stuff - doesn't make sense if either lhs.is_real is not True or rhs...  But I think, expr.py code broken in this sense as well.