units: exclude gettys from isolate requests

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.