vt(4): Always call vt_window_switch() in vtterm_cnungrab()

[dumbbell]

Summary

Why

This ensures that vtterm_cnungrab() is the mirror of vtterm_cngrab(). And the latter always call vt_window_switch() and thus the backend's vd_postswitch().

This makes sure that whatever the backend did during vtterm_cngrab(), it can undo it during vtterm_cnungrab().