(max_population): New, factored out of max_pop().

(max_population): New, factored out of max_pop().
(max_pop): Use it.  Don't support null argument anymore.
(nati, gen_power): Use max_population() instead of max_pop() with null
argument.
(show_sect_stats): Use max_population() to simplify the code.
(prod, upd_buildeff): Use max_population() to get correct new maximum
population for limiting work when tearing down a big city.
Previously, this was broken in upd_buildeff().  In prod(), it was
broken when the new designation had odd maximum population, which
should not occur.