update neweff production: Limit work in big cities

update neweff production: Limit work in big cities

Civilians, military and uw work only up to their sector's population
limit.  The population limit depends on the sector type's maximum
population, research if RES_POP is enabled, and the sector's
efficiency for big cities.

The population limit may decrease between computation of work in
do_feed() and the end of the update:

* Research declines (only relevant with RES_POP).  Work is not
  corrected.  The declined research will apply at the next update.

  Since levels age after production is done, any work corrections
  could only affect leftover available work.  Wouldn't make sense.

  The effect is negligible anyway.  Even with an insanely fast decline
  of 60% (level_age_rate = 1, etu_per_update = 60), the population
  limit decreases by less than 10% in the worst case.

* upd_buildeff() changes sector type and efficiency.  Work is
  corrected only when this changes the sector type from big city to
  not big city.

  It isn't corrected on other sector type changes.  These can affect
  maximum population since the sector type's maximum became
  configurable in commit 153527a (v4.2.20).  Sane configurations don't
  let players redesignate sectors to a type with different maximum
  population.  The server doesn't enforce this, though.

  It isn't corrected when a big city's efficiency decreases, but
  sector type change isn't achieved.  Harmless, because tearing down a
  city takes very little work (25 for 100%), so efficiency decrease
  without type change means the work we have must be safely below any
  sane population limit's work.

Good enough.  However, the code implementing the work correction for
big cities is unclean.  Get rid of it by tweaking the rules: a big
city's extra population does not work.  City slickers, tsk, tsk, tsk.
At least they still pay their taxes.

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