(pchrstr, MAXPRCON): Simplify variable-style storage of constituents.

(pchrstr, MAXPRCON): Simplify variable-style storage of constituents.
Store only up to MAXPRCON constituents, not MAXCHRNV; code doesn't
fully support more than three anyway.  Remove member p_nv, use item
type I_NONE for unused slots.  Rename members p_vtype, p_vamt to
p_ctype, p_camt to avoid confusion with variable-style storage.
(pchr): Initializers adapted.
(nullify_objects, materials_cost, materials_charge, prod,
show_sect_capab, grind): Adapt, simplify where possible, protect
against bad item types in pchr[].
(MAXCHRNV): Unused, remove.