Sectors need space for items, deliveries and distribution thresholds.

Sectors need space for items, deliveries and distribution thresholds.
To save space, the ancients invented `variables': a collection of
key-value pairs, missing means zero value, space for `enough' keys.
This complicates the code, as assigning to a `variable' can fail for
lack of space.  Over time, `enough' increased, and for quite some time
now `variables' have been *wasting* space.  This changeset replaces
them, except in struct mchrstr, struct lchrstr and struct pchrstr,
where they are read-only, and will be replaced later.  It is only a
first step; further cleanup is required.  To simplify and minimize
this necessarily huge changeset, the new item[] arrays have an unused
slot 0, and the old variable types V_CIVIL, ... are still defined, but
must have the same values as the item types I_CIVIL, ...