SideProperty: next-numbers utype-value-list
This property gives the next serial numbers that will be assigned to units
acquired by this side.
Defaults to 1 for each unit type (Dijkstra notwithstanding,
that's still where people start numbering things).
If the unit is of a type that gets numbered
(assign-number property is true),
then any unit of that type, acquired by any means whatsoever,
will be assigned the next-numbers value for that type
and next-numbers will be incremented.