`stop_remote_server` should return `True/False` depending was server stopped or not

[pekkaklarck]

Currently this method always returns True simply because all XML-RPC methods need to return something. This should be changed so that True is returned only if stopping server was allowed and False should be returned otherwise. After this external tools stopping the server can know was the server actually stopped or not.

As a separate task outside the scope of this project, the remote library API documentation needs to be updated to recommend that stop_remote_server returns True/False.