(get_materials): Redesign.

(get_materials): Redesign.
(shiprepair, planerepair, landrepair): Use it.  Behavioral change:
ship repairs outside harbors and plane repairs by a carrier can use
fewer materials.  Before, such repairs consumed each required
commodity as far as available.  Now, they consume the same fraction of
the real cost of each commodity, i.e. commodity use is limited by the
most scarce commodity.  Neither old nor new behavior make much sense,
but the new code is simpler.