units: exclude gettys from isolate requests
gettys are nowadays mostly autospawned and hence usually subject to being shut down on isolate requests, since they are no dependency of any other unit. This is a bad idea if the user isolates between multi-user.graphical and graphical.target, hence exclude them from the isolation. This has the effect that gettys no longer cleaned up when emergency.target is isolated, which might actualy be considered a feature, even though it is a change from previous behaviour... Note that the one getty that really matters (the one on tty1) is still removed when isolating to emergency.target since it conflicts with emergency.service.