Fix a channel price that is wrong on tax or display unit

Recognise a seeded custom channel price and correct it instead of editing the default price.

1. A channel that looks right on price but wrong on tax or display unit is a custom price that was seeded from older default values.

2. Changing display unit or tax on the default price will not fix custom channel prices — each must be edited in its channel or re-linked to the default.

3. Field defaults when never set: tax 0, margin 0, display unit per-unit, discount 0 percent, maximum percentage discount 100.