(P_SHELL, P_GUN, P_PETROL, P_IRON, P_DUST, P_BAR, P_FOOD, P_OIL)

(P_SHELL, P_GUN, P_PETROL, P_IRON, P_DUST, P_BAR, P_FOOD, P_OIL)
(P_LCM, P_HCM, P_TLEV, P_RLEV, P_ELEV, P_HLEV, P_URAN): Use -1 instead
of 0 for `no product', shift down product indexes and update
product.config and sector.config accordingly.
(budg, prod, show_sect_capab, produce, produce_sect): Adapt test for
no product.
(ef_elt_byname): Remove the hack to hide pchr[0].
(prod_eff): Oops on no product.