It was horribly broken before. :) Only the first password set via
`sv_rcon*_password` worked, and that only due to a bug in `AddHashKey`
-- `AddHashKey` incorrectly assumed that only key slots greater than 0
are valid, but 0 is valid too.
Fix default passwords on password removal, fix no admin password being
generated if you only had `sv_rcon_mod_password` or
`sv_rcon_helper_password` set. Add an assert that would have caught our
mistake.