budget: Fix treasury tracking

budget: Fix treasury tracking

The update simply updates each nation's nat_money as it goes.  Works.
Except it doesn't update when it runs on behalf of budget.  But it
still checks nat_money to determine whether the nation is solvent.
These checks are all broken.  Leads to massive mispredictions when
you'd go broke or solvent during a real update.

Track money unconditionally in nat_budget[].money.  Delay update of
nat_money until prod_nat().  Replace separate money[] by new
nat_budget[].start_money.  Closes bug#235.

Remaining difference between budget and update in the update test:

* #1: budget mispredicts plane #100 gets built (to be fixed)

* #2: budget shows ship, plane and land unit maintenance when broke,
      but update damages them instead (correct)

* #2: sector -14,0 converts, quadrupling its taxes (correct)

* #4 & #5: bank with dust and bars taken over by che (correct)

* #4: plague deaths (correct)

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