Add support for multiple admin passwords

alright, let's be real here: having a single master password that every moderator have to share is not good design. all you'd need is a single moderator to turn against you or simply do something stupid and you're screwed. recovering is going to take a while too if you have multiple moderators, since the password need to change and all moderators need to be informed about it, which is also made worse by the fact that sayto is broadcasted to all players (seriously??).

anyway, there's simple solution: allow multiple passwords. with this patch, password now adds a new password instead of replacing the current one, and clearpassword removes all of them in case it's necessary. while there's obviously room for improvement here, this should at least be good enough to fix the core problem.