sd-bus: rename sd_bus_open_system_container() to sd_bus_open_system_machine()

sd-bus: rename sd_bus_open_system_container() to sd_bus_open_system_machine()

Pretty much everywhere else we use the generic term "machine" when
referring to containers in API, so let's do though in sd-bus too. In
particular, since the concept of a "container" exists in sd-bus too, but
as part of the marshalling system.