Resolve more simple critical TODOs

Resolve more simple critical TODOs

- Rename `file_exists()` to `file_stat_errno()`
- Make result string `node2file()` consistently free-able
- Move `valid_file_or_dir()` to file.c, rename to `file_is_valid()`
- Make cache threshold configurable
- Recover from errors in `__vfprintf()`
- Remove recursion from `cer_cleanup()` and `cer_free()`
- Delete snapshots and deltas after exploding them