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/prepare.c

@@ -44,7 +44,7 @@
 #include "update.h"
 
 void
-prepare_sects(int etu, int *bp)
+prepare_sects(int etu, struct bp *bp)
 {
     struct sctstr *sp;
     struct natstr *np;