Security check when lobbyboy server starts.

Need to check:

- user started lobbyboy with default username/password?
- with the ssh host key https://github.com/lobbyboy-ssh/lobbyboy/blob/main/dev_datadir/ssh_host_rsa_key in git repo (middle man attack)

If yes, then need to print warning information to stderr.

related discussion: https://github.com/lobbyboy-ssh/lobbyboy/pull/32