Server: Removed "net server start/go/stop/close" commands

The server is always "started" when it's running; quitting the server
means closing it. In other words, it is no longer possible for a
server instance to be running without being in the "started" state.
Security|Server: Disallow public servers without a shell password

If there is no shell password, anyone can login via Shell and modify
the server's config.

It is now mandatory to set 'server-password' before announcing the
server to the master server.