Link share fallback when demand is zero
Link share fallback when demand is zero

Extends the special-case when a link and associated slot have zero
demand to set their share to 0.0 (or 1.0 when it is a solo link), from
`constant` links to all except `flexible` and `share`.

quintel/etengine,quintel/etengine,quintel/etengine,quintel/etengine