bus: move ssh support into public API of libsystem-bus
bus: add API call to create bus connection to the system bus of local containers

Also, add support for this to machinectl, so that we can enumerate the
machines that run inside a container. We must go deeper!
machinectl: we don't really just show information, we execute operations
udev: config-link - one less log_info
udev: rules - move parsing error to log_error()