(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.
