bug#13580: 24.2.92; regression in calc-convert-units

[Jay Belanger]

> Please recognize the difference between dimensionless and unitless.
> 3 ft/in is dimensionless, but it has the unit "ft/in". 

"The expression is unitless when simplified"

> If the user entered "kg" as the old unit of "3 ft / in" this makes no
> sense.

Of course it does, and Calc has supported this behavior for years.
When Calc asks for Old Units:...  New Units:...,
it is regarding the expression on the stack as the coefficient of the
old units.