- `prepare_host()`: split; do checks earlier and allow them to be interactive
- introduce `exit_if_countdown_not_aborted()` for "Low Disk Space" and other critical conditions
- split `prepare_host()` into interactive & non-interactive parts
- split off `clean_deprecated_mountpoints()` from prepare into `cleaning.sh`
- introduce and use `reset_uid_owner_non_recursive()` for `.tmp` reset in trap, to avoid disasters
- add more logging sections to default-build.sh, avoid unlogged parts
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