Consistently consider a nation bankrupt when its treasury is

Consistently consider a nation bankrupt when its treasury is
negative.  Some places considered $0 as bankrupt, some didn't.  Fix
the ones that did:
(repo_list): report command misreported countries with $0 as broke.
(init_nats): If you had $0, logging out and back in bankrupted you.
(produce_sect, upd_ship): Failed to build sectors and produce stuff
for countries with $0.