(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.

src/lib/commands/powe.c

@@ -329,7 +329,7 @@ gen_power(void)
 
 	if (opt_RES_POP) {
 	    np = getnatp(i);
-	    maxpop = max_pop(np->nat_level[NAT_RLEV], 0);
+	    maxpop = max_population(np->nat_level[NAT_RLEV], SCT_MINE, 0);
 	    f = 1.0 + (((float)maxpop) / 10000.0);
 	}
 	powbuf[i].p_power *= f;