(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/nati.c

@@ -95,7 +95,7 @@ nati(void)
     pr("     Plague factor : %6.2f%%\n", pfac);
     pr("\n");
 
-    poplimit = max_pop(natp->nat_level[NAT_RLEV], 0);
+    poplimit = max_population(natp->nat_level[NAT_RLEV], SCT_MINE, 0);
     pr("Max population : %d\n", poplimit);
 
     safepop =