Use the new Empire clock for generating BTUs:

(natstr): New member nat_access.
(cou_ca): New selector access.
(grant_btus, accrued_btus): New.
(prod_nat, init_nat): Use grant_btus().  BTUs are now made at the
update in addition to login, because that lets us get away with a
simple ETU stamp (nat_access).
(nat_cap): Replaced by grant_btus(), remove.

src/lib/update/nat.c

@@ -133,6 +133,8 @@ prod_nat(int etu)
     struct natstr *cnp;
 
     for (n = 0; NULL != (np = getnatp(n)); n++) {
+	grant_btus(np, etu_per_update - np->nat_access);
+	np->nat_access = 0;
 	if (np->nat_stat < STAT_ACTIVE)
 	    continue;
 	/*