process imaginaries to determine whether expr is positive or negative

pairs of imaginary factors change the sign of an expression; detecting these was added to Mul._evalf_is_negative.
> Not the presence of sign.has(c.sign) I hope.

That was the idea.

> Because even though we don't know what the sign is we might know if it is nonpositive or nonnegative.

Hmm.  Any example?  I thought, all such cases "know" the sign.eval.