Use newly gained access to the current user's authentication state and
the command handler's auth_required flag to give correct 'commands' and
'notcommands' output to unauthenticated users when password
authentication is activated.
- Rename .frontend.MpdFrontend to .dispatcher.MpdDispatcher, as it
dispatches requests.
- Move exceptions into .exceptions.
- Move .server.MpdSession to .session.MpdSession.
- Move handle_pattern to .protocol.handle_pattern.
