Basic Account Flags

This changeset introduces some basic account flag functionality.

- The new `modacct` command will show an account's current flags and toggle them on and off.
- The new `restrict` command will restrict logins to administrators and beta testers. This is useful for update verification. A new `--restrict-logins` command line option is now available as well.
- The admin flag will allow a client to bypass the `makeSafeForNet` message filter.
- The banned flag does just what it says.