This commit adds a one-time password system. Users can generate
passwords by using the `genotp` command (or manually queuing a set_otp
event with the SHA-256 hash of the one-time password they want to set)
and these passwords will be valid, along with the normal password,
either until the computer shuts down (or the net daemon is killed), a
new one-time password is generated, or the one-time password is used.
