dbus: methods return chan, do not block

Allow callers to ignore the result of jobs corresponding to unit methods by returning the response channel. Doing this allows fleet to stop caring about the result of jobs and work around https://github.com/coreos/fleet/issues/903.
https://github.com/bcwaldon/go-systemd/commit/e6c5c0ec428aeb62dc4175afe59b74baf796ae56