power: Include sector materials and cost in power factor

power: Include sector materials and cost in power factor

Building sectors can make you rate *lower* on the power chart, because
the power factor treats all sectors the same, regardless of build
materials and cost.

To avoid that, replace the term

   efficiency / 10.0

by

   (power value of materials + power value of cost + 9)
   * efficiency/100.0

The value of ordinary sectors, which take no materials and cost $100,
doesn't change.  The stock game's fortress is now worth 80% more due
to its materials and higher cost.  The stock game's wilderness is
worth 10% less, because it costs nothing.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>