Replace the revolting build pointer data structure by a proper data

type.  Make it abstract because that's possible.  Change data layout
so that the slots belonging to a sector are together in memory, it's
nicer to the cache.
(bp): The new type.  Users changed.
(get_wp): Update accordingly.
(alloc_bp): New.
(update_main, calc_all): Use it.  Before, calc_all() allocated 1/7
more than necessary.

src/lib/update/human.c

@@ -55,7 +55,7 @@ static int babies(int, int, double, int, int);
  */
 int
 do_feed(struct sctstr *sp, struct natstr *np, short *vec,
-	int *workp, int *bp, int etu)
+	int *workp, struct bp *bp, int etu)
 {
     int work_avail;
     int starved, sctwork;