Add "1" at the start of all fraction-like units when required

If a single unit is given in the denominator then a "1" is needed in
all cases. There is an issue when we get to the numbers part, but
that can be dealt with later.
Tidy up (again) "unit/qualifier-mode" option

Somehow the code got mixed up here!
Add a leading "1" when needed with "unit/per-mode = repeated-symbol"

If the first unit is in the denominator then you need to add a "1"
and "/" here.