core/unit: refuse to spawn units under frozen cgroup

core/unit: refuse to spawn units under frozen cgroup

Currently, Unit.freezer_state is always initialized to
FREEZER_RUNNING. While realizing cgroups for frozen units
was disabled in 7923e9493c48694b32d1a6de7b9a996c0194bf17,
the commit only checked for freezer_state of the unit inself,
meaning that newly-loaded units might be started and the kernel
would hang pid1 when trying to spawn sd-executor into sub-cgroup.
This can be easily reproduced by the following:

```console
# systemd-run --slice=test.slice sleep infinity
# systemctl freeze test.slice
# systemd-run --slice=test.slice sleep infinity
```

Therefore, let's correctly initialize Unit.freezer_state
based on the parent slice.