kboot: Add host_exit and use it to implement exit()

Clients of libsa are expected to implement exit(). The current exit just
loops forever. It is better to really exit: when running as init that
will reboot the system. When not running as init, other programs can
recover (not that we support running as init, but when we do in the
future, this is still the rigtht thing).

Sponsored by: Netflix

(cherry picked from commit f56d7a73be6688299c7ec97fac6d505c657aa55c)